Build it yourself!!
You can do it in an afternoon, and you get an enormous sense of "I made that, and it works! Woo-Hoo!!" when you see XP boot for the first time. (especially your first build)
It really isn't difficult, but it might be an idea to be near a pc that's connected to the net, just in case you have a problem... Just google what you can't get working, and you're usually back on track in about ten mins...
Before you start Google for "AMD athlon64 PC building guide" or whatever matches what you bought, print it, and go!
You'll enjoy using the PC much more if you built it yourself, plus, it's CHEAPER!
And don't tell me you're worried about not getting tech support if you don't get prebuilt... www.techist.com
is all you need!