There is an interesting post here and a detailed discussion here of a problem that has hit some users of Microsoft OneCare.  Apparently, OneCare has deleted some folks' Microsoft Outlook .PST file, destroying all of their e-mail.  There are some work arounds in the post which describes how to make OneCare stay away from your .PST file.  This is a problematic solution, because OneCare won't be able to scan for any malware in your .PST file, but it sure beats having all of your e-mail deleted!