Sophos Anti-Virus custom action install log, located in the folder C:\Windows\Temp\, shows the following error...
Known to apply to the following Sophos product(s) and version(s)
Failed to grab execution mutex. System error 258.
Sophos Anti-Virus for Windows 2000+
The registry string PendingFileRenameOperations is currently populated and the computer is waiting to be restarted.
What to do
Reboot the computer and then run the Setup.exe program from the share (CID).
A further reboot, after installation, may also be required.