If I were looking at the difference between the full version and an upgrade, I'd get the upgrade version and buy a new hard drive. Then install XP on the new hard drive and point it to the old one when it asks for your proof of Win98-ship.
FWIW, I've just built a new PC, and it's my first one running XP. I went for a clean install... no possibility of old stuff giving you problems. [img]smile.gif[/img]
And on a side note, in the first hours of just setting up, Ad Aware and Spybot both found things they wanted to remove for me... [img]smile.gif[/img] They really are your friends, and you really want to use them