similar thing happened to me. i tried all i could but finished up restoring to a previous point. seems to have worked for me.
After receiving a popup advertizing anti-spam/virus/security software, my Windows desktop seems to be permanently changed.
We can no longer see or set the background wallpaper.
Everything else seems to be working OK.
The first symptom was the desktop seemed to be covered by one of those Security warning, 'someone is watching you 'the advertisements. We ran an Avast scan which identified a virus and removed it. After that, all we have is a blank screen as the background of our desktop. All the desktop icons are there, but we can't set a new backround. However, at shutdown or startup, the backround does show up for a few seconds before being replaced by the blank area. It's as if some program is running which overwrites it. I believe some default file was overwritten, but don't have a clue what to look for or how to recover it.