Ebay Bought PC w/WinXP pro-CanNot Uninstall!!!!

FIRST PROBLEM:
About 6 months ago,we bought this PC through Ebay... It
came with WinXP Pro installed along with a plythera of
other very cool programs. Problem: No Installation disk
and when we tried to "Uninstall" it says it's missing
files and can't. But we have just been limping along
through all it's problems since. (Its our second PC in the
house)
SECOND PROBLEM:
"HE" did not listen to me about doing "Windows Updates"
nor did he ever get the free "Spyware remover" I told him
about. Now, it won't stay on line long enough to download
the up-dates. The "Bullgaurd" anti-virus we added to his
PC under my account, won't finish scanning before it
freezes up the system. And the half it does get scanned
informs us he has "several" (5 - 7) Trojan.Downloaders of
a few various kinds infecting his system, and Bullgaurd
says, "Disinfection failed" and then "move failed" for all
the ones it shows.

There are numerous OTHER PROBLEMS and reasons why I think
we should just dump this XP Pro version and install my
98SE that I already own into it... PLEASE HELP ME FIGURE
OUT HOW TO REMOVE THE XP?!?!?!
Thankx in advance,

DESPERATE TO GET MY PC BACK TO MYSELF...~Angel