Could you perhaps give the specifications of your computer, perhaps hard drive model? I've seen a lot of problems with hard drives going out, and subtle things happen like a certain file becomes corrupted. Or, you could go to your Program Files directory and make sure the executable is in it's place. I've also seen where, if someone else has access to your computer, they poke around random files and delete them. Any one of these are a possibility, as well as administrative issues.
Also, have you scanned your computer for viruses or spyware? Some of the nasty ones can lock out access to certain EXE's, too.