have you tried to uninstall & reinstall the game? updated with latest patches?
Not sure about Vista, but in XP the saved games is in the My Games SubFolder in My Documents Folder... back that up, & after uninstall & reinstall, place it back in, your saved games should be intact.
have you done anything as far as software in the past week or so, that may have caused it? Updates on the PC, or something?