first thing to try
1. Login to Windows 7 with Administrator Rights.
2. Type ‘gpedit.msc’ in the Start menu search box.
3. This will launch Local Group Policy Editor. In the left pane go to:
User Configuration –> Administrative Templates –> Control Panel –> Personalization
4. In the right pane, double click on “Prevent changing desktop background
change to not configued (if not already)
or go to control panel and select another them and then change it back ( you will see pre saved built in windows themes, different wllpapers sounds etc, click on one and change it back
let me no how u get on