I have been working a project for a week now...and all of sudden I get the message docfile is corrupt while doing a save.  Model and parts still ok.  Then on subsequent saves same message...docfile is corrupt...  but the save seemed to be OK.  Then about 45mins later on model cannot save.  I shut down solidworks and restart...all my changes were gone..and during every save I get docfile is corrupt message and document is saved but when I open it up no changes I made are shown.  What is going on here?

I am running 2009 sp4.1.