I know it's a little late to be replying to this post, but it was the first hit when I did a search for "unable to right-click desktop" in google, so I thought it would be nice for the answer to be included in ths post.
First of all, you are going to probably have to remove whaterver spyware/virus that caused this first, before trying to first the desktop issue. the free version of AVG will probably work fine (
http://free.grisoft.com/). If the spyware/virus is not removed first, it will most likely just recreate this issue after you fix it.
In my case, I got infected with spyware by visiting a website and both AVG and Microsoft Anti-Spyware removed the spyware immediately after I was infected. However, my wallpaper had been replaced with a message explaining that I had been infected with spyware, I was unable to right-click my desktop (although I could right-click anything else) and when I went to the desktop control applet, the area to choose a different background was greyed out.
I found some information on policies that could be set for ActiveDesktop and Explorer that could cause this. These are located in the registry at the following location (click START-RUN and type REGEDIT to open the registry editor):
HKey_Current_User>Software>Microsoft>Windows>CurrentVersion>Policies>
Under Policies, there are 3 subfolders; ActiveDesktop, Explorer and System. In my case, the spyware that had infected me had added a registry key to the SYSTEM folder. This folder is typically empty to begin with, but I had a key called DESKTOP and the value was set to the location of the wallpaper that was being displayed (C:\Windows\Desktop.html). Once I deleted the DESKTOP key I was ale to right-click my desktop and change the background.
If you do not have the DESKTOP key or some other suspicious key in the System folder, I would recommend that you check the ActiveDesktop and Explorer folders for suspicious keys as well. For instance, there is a key under ActiveDesktop called "NoChangingWallpaper" that normally has a value of 0. If this is set to 1, just change it back to 0.
Typically I wouldn't recommend just deleting entries in the registry, but I can also assure you that anything can be deleted under any of the Policies folders of the registry and Windows will run perfectly fine. So if you see something that looks a little suspicious under any of the three folders, feel free to delete it. There is a similar Policies folder under HKey_Local_Machine, but it is less likely that you will find something there that would cause this issue.
