Some of the “fake antivirus” trojans that seem to be popping up everywhere you turn break things, even after they’ve been removed. Some of them cause an inability to launch programs – any program you try to open, causes a “can’t locate such-and-such file”.
Some fixes that may help:
http://social.technet.microsoft.com/Forums/en/itprovistaapps/thread/a8e61d1d-e1d7-421e-94de-7d0efc49aa82
http://social.answers.microsoft.com/Forums/en-US/vistarepair/thread/a7bb0940-703c-424e-824c-85d5bd83d1e9
http://www.winhelponline.com/articles/105/1/File-association-fixes-for-Windows-Vista.html