Try your local computer store for laptops. they usually have all the parts. I generally see them for about $200.

Here are some desktops and laptops with (probably pristine installation of) Windows XP Pro or Windows 2000 Pro on it averaging $200-300. If you don't care for having an OS on it, here are a bunch of them and here are some XEON boxes, some of them dual-procs. laptops run about $330-430. wow. maybe you should get one new (you get OS, driver & app cd's) for $500. but then again maybe these aren't so bad - they have XP. take a look at the features. Laptop drivers are going to be a bear, because for one model of laptop there is going to be multiple models of ethernet, multiple models of modem, multiple models of audio to choose from, etc. and knowing which you have is critical. Better write down what you have when you start. Pretty much same goes for a desktop too.

Quiet your PC if you so desire: try a Zalman CNPS7000-Cu or CNPS9300 AT or one of their many other silent CPU coolers (they make VGA coolers too) or a Thermalright. There are specialty stores such as Directron and Silicon Accoustics. you can probably get them from nextag.com. Find and replace the loudest part in your case.
Definitely check out silentpcreview.com.

for desktops, I was given this link to these no-OS $200 Dell Optiplexes
DELL GX270 SLIM P4 3.0GHz 120GB 1GB DVD+RW (qty in stock: 1).
DELL GX270 TOWER P4 2.8GHz 120GB 512MB CDRW/DVD (qty in stock: 1)
DELL GX270 SLIM P4 3.0GHz 120GB 512MB DVD (qty in stock: 18).
You can purchase an OS for these for about $370.

The craigslist.org route ($100-200)

Questions to ask:

• how fast is it? (MHz is slower, GHz is faster) . (for XP, you will need at least a 2.0GHz machine to be tolerable. for Vista, get it as fast as you can and make sure it's a dual-core.)
• Is it Dual-Core or AMD X2 (dual-core)? If you can get either of these or a quad-core, it will give you a big perormance boost. Only applicable to 2000 Pro/XP Pro/Vista.
• Ram: how much memory (RAM) does it have? (for 2000/XP you want 512MB-1GB RAM minimum, 2GB minimum for Vista)
• Optical drive(s): do you want a DVD burner or DVD-rom rather than a cd-rom? DVD burner DL? cdrom drive? DVD-ROM? DVD-ROM/CDRW combo? If the case is one of those flattened Slimlines, does it use standard-height 5.25" drives (1.6" tall) thatr you can upgrade? My preference is a DVD Burner. If you get a Blu-Ray burner, that is the best.
• Hard Drive(s): what size of hard disk? (80GB minimum for XP, 120GB minimum for Vista). XP Pro with added SP3 takes 9GB. Vista is max. 7x bigger (42GB). My mother uses 55GB on her XP laptop. I use 387GB as a software developer on my XP Pro SP3 box. Pictures and video, and music tend to take up mounds of space. so get a big disk if you are going to be doing a lot of that (minimum 320GB). you can always mount it as a second disk. see moving My Documents article.
• Video: does it have a video card, and if so, what kind? Is it DirectX 9? If you plan to run Vista in the future, is it Vista certified and does it have at least 256MB video RAM? More and more Vista-compatible software like Roxio 9,10 and Nero 8 are requiring a real video card instead of just built-in video on the motherboard.
• OS: does it come with the 2000/XP/Vista cd or the System Restore cd(s) (most of the time the answer will be no, but be very happy if they say yes). 2000 is no longer supported. XP Home means you are dealing with a single-core system. XP Pro means you are dealing with possibly a single or dual-core machine, probably dual-core. Vista machines are usually dual-core or once in a while quad. Having the OS cd's means you can rebuild if the system gets corrupted because of spyware.
• Drivers: does it come with driver cd's? (most of the time the answer will be no, but be very happy if they say yes)
• Printer: does it come with a printer (and do you want one)? try to make sure it comes with the printer software cd if you can.
• Monitor: does it come with a monitor (and do you want one)? if so what type and size of monitor (LCD or CRT)?
• KB+Mouse: does it come with a keyboard and mouse (and do you want one)? (again, this is up to you - maybe you already have a set of these). [these are cheap to buy - $40 total] . make sure that, if you use your own, they are compatible with what is on the machine - newer machines only have USB ports, no PS/2!
  Note: a wireless mouse/keyboard will require batteries or rechargeables, so make sure you have enough on hand. If you use rechargeables, have 2-3 numbered sets (1, 2, and 3), and a top-end smart charger. A smart charger like the BC1HU will charge each cell separately and indicate dead cells (each cell has its own individual charging characteristics).
• Office package: If you require Microsoft Publisher, you need Microsoft Office Pro 2003 or 2007 or you can always purchase Publisher separately though, but it's expensive. If you just need any publisher, try the free Scribus. It requires ghostscript/postscript.
  If you can get by with an Microsoft-Office 2003-compatible package like the free OpenOffice.org, try that (it has no publisher yet, and it's different in the way you use it). Scribus has the advantage that the files are text-editable if anything goes wrong. MS Publisher has the advantage that Print Shops often support that file format.

you can get a used XP machine off of craigslist.org for about $100-150. I wouldn't go under that price because you get junk or an illegal OS or both.
AV: You will also have to clean off the spyware and viruses pretty much, so pick up a comprehensive antivirus+antispyware+firewall package like *Microsoft OneCare*, Norton 360, Mcafee Total Protection, Kaspersky Internet Security, (Norton cd's have the advantage of being able to boot off the cd and clean the machine if nothing else works, but only if you are using FAT32 filesystem). Then clean the machine by doing a scan or sweep or both. it will probably request a reboot to finish a cleaning. do the scan again until no reboots are left. scans take about 2 hours each (2 days for 163GB of files - chances are it won't be that much). If you don't want to do this you can take it to a local computer shop and have them clean it for a fee, in which case you may end up with software (maybe you can bring your own).AV:
Before you install the new antivirus/security package, you should check to make sure if it already has one. But what you have might not include spyware protection (sometimes people don't update their package for years!). All of this year's programs include spyware protection. You may have an AV package dedicated to someone's internet account (email address+ISP) like Mcafee does. Check the subscription status to see if it is expired. if it is, uninstall it and replace with Microsoft OneCare or Mcafee Total Protection or Norton 360. If it is OK (your subscription is going to run out sooner than 1 year), either remove the old one and install a new package, or leave things the way they are until they expire. If your AV protection is provided by your ISP and you wish to use that instead, you will still need to uninstall the old package, even if it is the same package (different email address, different subscription!).
Note: prices on AV software go down the more time wears on!

Optical drive: try to get one with a DVD burner (preferably a DL DVD Burner) or at least a cd burner if you can.
Monitor,Peripherals: pick your machine wisely. It should come with keyboard & mouse if you don't already have one - the monitor and printer is up to you. There are usually some 17" CRT monitors floating around for <$25 (your video card may require a VGA-to-DVI adapter if you have a 1.5" wide rectangular DVI connector with lots of pins in a grid fashion and a plus-looking thing).

With a used machine you are not likely going to get the software CD's/DVD's that came with it. The drawback with this is that if ever you need to rebuild your machine, you won't have the XP cd or the software... XP Pro is $269 retail. XP Home is $192 (while you can still get XP - see EOL below).

Vista uses 40GB of disk space all by itself, not to mention updates... my XP Pro with added SP3 takes 9GB. but with all the data a person puts on their computer (and progams), you should expect to require about 80gb for XP and 120GB for Vista. My mother uses about 55GB. I as a software developer use 387.12GB.

EOL: XP Retail is scheduled to be EOL'd (End Of Life) on June 30, 2008 (but it is still available is some online stores). That just means it will no longer be sold it will still be supported. XP Service Pack 3 is coming out in 2008. But system builders can build XP systems using the OEM version until Jan 31, 2009. This is not the first time microsoft has set an end-of-life date for XP. the first time it was only 5 years, which would have made it 2007 for XP. but people are still buying it after they get vista... so now it's mid2008 and early2009.