I was working on a server that was running low on disk space on the system (C:) partition.  I was able to free up some space rather quickly (by removing the Automatic Update downloads), but when I checked the Event Logs, the Application log was filling up with errors from SMS for Exchange.  The message was that the virus definitions were corrupted.  It appeared that the XDB down script had run around lunch time and updated the virus definitions, but wasn’t able to complete the install due to low disk space.  Despite the partial install, SMS for Exchange appeared to be trying to use the corrupted definitions.  When I tried to run LiveUpdate (as recommended by the Event Log message), LiveUpdate said everything was current.  People were starting to have problems with their e-mail (and for some reason the server was beeping irregularly on site).  I stopped the SMS for Exchange service (which fixed the e-mail and the beep), but the service wouldn’t restart.  I tried restarting the main Antivirus service as well, and it would not restart (also because of corrupt virus definitions).  I had to manually stop all the Symantec services, remove the partially installed virus definitions from the C:\Program Files\Common Files\Symantec Shared\VirusDefs folder, manually edit the USAGE.dat file (which tells the Symantec products which defs to use), then restart the services.  Once the services were up and running on the previous virus defs, I was  able to re-run the XDB down script and let it update the defs to the most current.