Look for one of those deals from DELL. (Score 1)

hang out at deal sites and get something from dell. here's a deal that was posted a few days ago at bensbargains.net
OptiPlex GX240 1.6GHz $358 shipped from Dell Business
Pentium-4 1.6GHz, 128MB SDRAM, 20GB Drive, 16MB Rage 128 Video, XP Home
Integrated 10/100 3Com NIC, 48x CD-ROM, Integrated Audio, 3-yr Warranty

This was done using a 20% coupon and a $100 rebate. Dell does these things kind of regularly and if you're diligent you can jump on one of these. The Optiplex is Dell's Business line of machines and the 3 year warranty for their business machines is pretty good. even without the $100 rebate those machines will only run you about $450. Sure it's not the best marriage of all components (sdram instead of ddr, lower end video card, xp home) but for that price you get some pretty good warranty and they're decent work machines.

If you wanna go for even less money, try compgeeks.com's specials, Get a grade A celeron 700mhz with 20gb hd, 64 or 128 mb ram, cdrw, keyboard, mouse, nic, modem, and it'll probably run you $300, just recycle your old monitors.

hang out at deals sites like:
http://www.bensbargains.net
http://www.fat wallet.com
http://www.anandtech.com