Have you recently installed any new MS software?
This can be a known behaviour of MS software if pirated copies are in use.
Had a friend who had a perfectly legal installation of MS office, the full monty, all working ok.
Then he loaded a bootleg copy of MS Project, and everything started behaving the way you describe - load flash screen, then drop out.
There is an entry on MS KE base which I can find for you if needed, let me know.