I have browsed this forum and others and have not find an answer to my problem.
When I right-click on the desktop, I got the proper Display Properties window.
All expected tabs are there (they never disappeared), everything can be changed (theme, wallpaper, screen saver, resolution, etc.) and no button is greyed.
Simply, changes never apply, either as click OK or Apply, and when I come back in the Display properties, all old values are back.
I already checked the registry under HKCU\Software\Microsoft\Windows\Current version\Policies and I got no active policy, no Active Desktop and so on...
I had no recent virus attacks and regularly run SpyBot Search & Destroy and Lavasoft Ad-aware.
What is strange is that I can select any image, right-click on it and select "Set as wallpaper", this works...
I do not know since when exactely this is the case, and what could have caused this. I know it worked a few weeks ago.
The only thing I can think of (apart from the games demo I installed recently ), is that I had two screens, but had to give one. But the system stills thinks they are two. I upgraded my ATI Catalyst Control Center and drivers to the last version, but to no avail. I have an ATI Radeon X300 card.
Thank you for your help, it's driving me nuts.