During installation of Sophos Anti-Virus for Windows 2000+, the following error is displayed:
The Windows Installer Service could not be accessed.
This can occur if you are running Windows in safe mode, or if the Windows Installer is not correctly installed.
Contact your support personnel for assistance.
Sophos product and version
Sophos Anti-Virus for Windows 2000+, version 7.3.1
Windows XP Professional SP2
What to do
Please refer to the Microsoft knowledgebase articles: