Problem: You cleaned all spyware/viruses but still cannot change background picture. Under display properties/desktop Background section is disabled.
Solution:
!!!DON'T FORGET TO BACKUP YOUR REGISTERY!!!
Remove following registry entry from your system using Regedit. Log-off and log back in.
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\System]
"Wallpaper"=""
You should be able to change your background now.
Good luck
M.Aydogan