OK i did something really dumb but cant figure out how to undo it!

I tried to open a program (while i was on the phone) and choose to open it as editor. no my computer tries to open everything as ms editor which make my machine useless pretty much i have to use the "open with" utility on xp to open any programs!

This is hell and some programs like the restore function doesnt work now as a result.

Please can anyone help?