If you’re receiving the above error and the account you’re using has the correct permissions, the issue could be related to a machine being cloned with Sophos installed and caused a duplicate machine name or SID issue.
Run the below script from a command prompt Window:
MsiExec.exe /i “c:\Program Files\Sophos\AutoUpdate\cache\savxp\Sophos Anti-Virus.msi” REINSTALL=ALL REINSTALLMODE=voums UPDATEDRIVERS=0 /l*v c:\msi.log /qb
On Windows Vista the command is:
MsiExec.exe /i “c:\ProgramData\Sophos\AutoUpdate\cache\savxp\Sophos Anti-Virus.msi” REINSTALL=ALL REINSTALLMODE=voums UPDATEDRIVERS=0 /l*v c:\msi.log /qb
If the above does not work, click the following link Cloning a machine with Sophos installed