UAC ON = "Windows cannot find.... ANY .exe"

Vista x64, fresh install, all updates applied and running great since about a
week. Today I logged and I can't start any ".exe" program. I get the
following message:

"Windows cannot find 'anyprog.exe'. Make sure you typed the name correcty,
and then try again."

I get this if I click on a file from Explorer or if I type a name in "Start
Search" from Start menu. Control Panel is dead for the most part.

I rebooted in safe mode and turned UAC OFF.

Everything now works fine when booted normally.

WHAT IS WRONG?

Thanks for any help.