If you also lost your task bar with the start menu something happened to explorer.exe or something is crashing it. If you have already restarted the computer, try pressing CTRL+SHIFT+ESC to get access to the task manager, if it opens up click on File > New Task then type explorer.exe and press enter or OK, if explorer was closed and not corrupted it should provide you back with your desktop at which point I would suggest to Check for viruses and if not suspected of a virus go ahead and do a system restore back to when the desktop was working.
If the desktop does not come back by running explorer.exe you can try one of 2 things.
1- Try going in to Safe Mode and see if you can do a system restore from there.
2- If not possible you will require your Windows CD to be able to do a repair of the OS with it.
Good Luck.
Mike