If it just stopped working all of the sudden you could reinstall everything on your computer by putting in your windows 7 CD your old files would still all be saved in a map called windows.old and it would most likely solve the issue.
Well it's always advised to keep backup files so if you don't already do that see to that you do because you can easily lose a file.
Are you experiencing any other issues on your windows machine besides the world editor or is it the world editor specifically?
Reinstalling didn't work?
What processor architecture do you have?
Is it 64 bit or 32 bit? (shouldn't make that much of a difference though)
Got administrator priviliges?
Tried compatibillity mode?
Update got it working again thx all