Hummmm good one, try this. when you are on the desktop with no icons opened up press CTRL-SHIFT-ESC this will open up the task manager, goto file then new task and run c:\winnt\explorer.exe -- note you must put in the full path. if that then opens up an explorer window i would check to see if there is an explorer.exe in the c:\winnt\system32 folder, if there is you have a virus.
there is also the option to try this. use task manager just like above and open up regedit then delete the key of: HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices -- then restart and the key will be re-built for you.
Good luck, and have fun.