So, I've decided that I'd like to get a second computer...because I'm tired of Rho fucking this one up.

The new one will be for Rho's internet journeys and music listening.
Rather than buy a new one though, I'd like to build one. I'm not the most mechanically inclined soul, but I watched UT build my current one (which is fucking awesome, BTW), I've looked at a couple of how-to sites and I have a bunch of geeks on a couple of different boards that will probably have no problem putting their $0.02 in. I think I can do it with just a little help.
Two questions to start:
--Is there anything that you all would not recommend buying used in putting together a computer?
--Is there a way to obtain WinXP on the cheap?