Error when installing Sophos Antivirus software

Hello Gurus!
I was trying to install the Sophos Antivirus software in my Win XP PC when I got this error message, see below.
I have searched the web and saw a way how to resolve the problem, but I was still puzzled over the resolution.
I knew I have the correct time and date in my PC.
Is there any other way to resolve the problem? Thanks.

Setup
Invalid manifest file: [VE-BADCERT]: 7
If you do not click ‘OK’, this message box will close automatically after 60 seconds.
