Not sure how it happened, but many files and applications suddenly will not open. Keeps asking me which program I would like to use to open such and such file, even to change time and date for example. Error message saying "Application not found" pops up for many Vista control panel functions.

Any thoughts?

Have you tried doing a System Restore to a point before when the problem started?

Yes. I have done this twice, just to be sure. Nothing changed.