Step into the Wayback Machine and check out this refurbished Compaq EVO D51S desktop. It's decently equipped: 2GHz Pentium 4 processor, 512MB of RAM, 40GB hard drive, DVD-ROM drive, and Windows XP Home. It even has a floppy drive! Geeks.com price: $99.99.
You'll have to supply the monitor, mouse, and keyboard, and you'll have to live with a case that has a few dings and scratches. (Read the full product description: This is definitely a "used" PC.)
Even so, this could be an ideal machine for young kids, tween/teen students, or a grandma who just wants e-mail. You could also turn it into a media server for your home network (though you'll probably want to add a larger hard drive). Not a bad deal at all for 100 smackers.