Do you have or at any time had a wallpaper with the text "Spyware Infection Detected" bla bla?
In that case: PROBLEM SOLVED.
Your desktop is blocked by something similar to a virus, so called "Addware". That is an add from a less serious, so called company, that is forcing their software on you. They plant that with the pretext that you need to scan your PC and when that is done you have to pay for the removal of something they themselfs planted on your computer.
If you remember when that happened, you can do a search looking for something called desktop.html or desktop.htm in your windows dir. Be sure to set the search on looking through the hidden files also. Look for the date when that file was created/changed, and if it corresponds with the date of the mishapp, just delete it. I suggest though that you first make a back-up of the file (by simply rename it .bak) before you delete it. Just in case you erase the wrong file.
Then go to Start > Run, type "msconfig" and click on OK.
Go to the Startup tab. Check so nothing named like the file/s you have deleted are on the startup list.
Un-ticking the checkbox next to a startup item will stop it from launching. [There are a few application that you will never want to disable (ctfmon comes to mind), but for the most part the best settings vary greatly from system to system.]
If you know your system well, you should then be able to desable the right programs from the startup and put a stop to all that trouble you are having.
Another way is to remediate the problem through changing a few registry entries in your registry editor.
I will get into that if you can not solve your problem by posted suggestion.
Good Luck Nekko