Problem with my Dekstop

[Teckademics8]

Alright so I go over to my friends house for a little halo. Then I deside to help him out with his computer since it is pretty slow. I get on his computer and get on his account (windows XP) and then I deleted a lot of spam and crap and got rid of it and ran Mcaffe and Ad aware and a few other progams to get it up to speed. But when ever I try to change his background it dose'nt work. It just changes and then it goes back to just being plain white. So it's not changing. Anybody have any ideas of what I could do, do get this to go away? Any feed back would be very apreciated. Thanks!