poor performance

I have windows xp media center edition and recently when I tried to view a
video from the media center program it wouldnt play the video. Also i
recently downloaded internet explorer 7 and everytime a new window pops up my
viruscan says its trying to change the registry. Everything involving the
internet slows my computer and sometimes some programs stop responding. My
computer didnt come with a disk, is there a way to fix the problem or restore
my computer to the original condition when I first bought it?
--
chad