Another one for NewEgg for purchasing your own parts to build your own. But I will suggest if you only use your computer for basic needs (internet, email, word processing) it is much cheaper to get a prebuilt one through someplace like Dell. Dell's prices for basic full computers (monitar, keyboard, etc.) with a legal copy of XP are unbeatable. How many of us can build a full computer with monitar, a legal copy of XP for less than $400? XP alone is $200.
If you do a lot of gaming or a lot of video processing and such, then build your own and it will be much cheaper.