This one seems to bug lots of Sophos AntiVirus admins.
When you install or try updating Sophos on a XP PC, the install tries installing but then fails. If you hover over the sophos icon located in the taskbar, it displays ‘update failed’.
The issue is highly likely to be related to permissions within the registry.
The issue may be related to a permission issue on the following registry key:
1) Locate the above key, if you get an error when clicking on the Windows key, you have located your problem. If you don’t chedck that everyone has not been denied rights to the key. If yes, continue with the procedure below.
2) Right click on the Windows registry key and click permissions you may find its empty. If it’s empty, click advanced and change the ownership to yourself.
3) Or You may find ‘Everyone’ with deny access, if this is the case click and remove the group ‘Everyone’ and click OK.
Right click the Windows key and if the permissions have been restored, the above key will show all of it’s permissions.
Try the install or update again