TBH, if it isn't the latest greatest PC you're after, you could probably acquire a lot of bits from people with newer computers...
I find its very easy to pick up something like a P3 for either free or very little from a friend or family member - for a few beer tokens perhaps - I was lucky enough to be given an IBM P4 by a friend. Loads of secondhand systems go on ebay, if you can wait a couple of weeks for one to come up near to you and cheap enough, there's always that option.
Or the other way to do it, is just upgrade what you've got bit by bit. This could also be done cheaply by keeping your ears open for people upgrading to newer equipment. Start by getting a better mobo + processor - incidentally, if you wanted to buy new, Novatech do nice mobo/CPU/RAM bundles. And at the end of the day, if there was one thing I'd buy new it would be the mobo, having done a bit of research and found the best one in my price range. Things like CD-ROMS can be upgraded later, if at all....