Dell Vostro 200 Minitower
Apparently it can be pretty cheap too: For $379, you can get a Vostro 200 Minitower for $379 -- regularly $629. And unlike most computers you find under the $500 mark, it includes a 19 inch flat panel monitor and all the software you need to get started: no Microsoft Word but you can probably make due with Microsoft Works, which is included.
It also comes with Dell's usual strong warranty/service deal and a keyboard and mouse. You'll need to buy speakers on your own but that's not a big deal: You can get those for $10.99.