Last week the COM+ folks released a critical update that has some unfortunate side-effects for people who have tinkered with the ACLs (security permissions) on their drives. Since Microsoft has been beating the security drum for several years now, the number of users who have tightened down the screws wrt their ACLs is probably quite large.
The COM+ update, and the attendant KB articles explaining the problems that can be caused, are very generic, though, so if you are impacted by this problem it's a little harder to link it up with the update. Basically you'll get a lot of spurious error messages in the event log about "inconsistent internal state" and "0x80070005" (which maps to a CRC error according to winerror.exe).
Who does this affect? Only the people running COM+. Who's that? Anyone running Media Center, for example. If you have this problem, the fix is noted on the KB909444 page; basically you have to grant Full Access permissions to System and Administrators to %windir%\Registration, and Traverse Folder (List Folder Contents) permissions to Everyone for %systemdrive%, and %windir% (including subdirs).