Software

either your user profile became corrupt, or it has had permissions taken away from it.

You can try resetting permissions from the administrator account
You can also try creating a new profile and copying the data to it

Start -> Control Panel -> User Accounts

Then find your user account and make sure that your permissions are set correctly.

Good luck.

I (thanks to a tip by someone here) tried to make another account with admin privileges. That account was able to write to its desktop. I then deleted that new account and at that point the old one is working. Thanks to all.