Right-click the desktop and sellect "Properties"
On the "Desktop" tab, make sure that, at least, you actualy have a walpaper sellected.
Maybe the image sellected is in fact solid white... or a very small one, maybe a single pixel. Try chosing another one.
Some of the best and most recomended antivurus:
AVG free
Avira
Microsoft Security Essentials
Install one of those and do a full scan.
Electro
May 2011