cpu - faster the better... 500mhz is slow... i run at 1.9ghz and its plenty fast for now... starting to get into gaming so i might upgrade soon
ram - if your gonna run winxp make sure its at least 512mb
hard drive - depends how much your gonna have on your computer... for most people a 40gb is well enough
thats pretty much the basics

theres video and sound cards too... but idk if i would worry too much about them if you dont know that much about computers
unless your a gamer or really love music - even so youll probably have to make an upgrade later anyway since most preassembled pcs come with cards to make your computer run good but not really performance or anything