|If the overlay is always there how can you tell that Task Manager is behind it? and how long ago did the problem start?|
The first thing I would try is a System Restore, it may be simply some malware that is set to take over explorer.exe, though I've never seen one that actually 'hides' your own desktop behind. Anyway, if you cannot use safe mode, which would give you the 'repair my computer' option then you need to use a Windows 7 recovery disk - if you don't have one already made then, assuming your new computer is Windows 7 you can create one using that which will be good enough to get to the repair system.
If you don't have access to a disk or another Win7 computer then it's a bit it of a longshot but if you are certain it is there and it only happened recently you can try running programs 'blind' by running Task Manager then press the 'alt' & 'F' keys, then press the 'N' key - this will open the 'new task' dialogue so type 'rstrui' and press 'enter' (which will start 'System Restore'), press 'enter' again which will choose the recommended restore point, then press 'enter' again which should be the 'are you sure' box and then see if the system does restore.
Incidentally does the overlay itself display any clues to what it is? - they normally try & con you into paying money.
"I've always been mad, I know I've been mad, like the most of us..." Pink Floyd