OK I need some help here with a server setup.

I have some financial limitations so if I can piece this together from spare parts it would be a good thing. doesn't have to be state of the art it simply needs to do the following

500g + drive space (easy enough)
Compatible with Vista, XP, Mac
Network print capabilities
and of course cost

Can't affor to buy a fancy schmancy server setup so this is to get us by for the next year or so until things balance out a bit better.

So what ya got as a suggestion?