Sophos Anti-virus problems

If you've seen most of my posts, I have many problems with my computer.

I think the reason why I get so much **** on my computer is because I don't have an anti-virus. So anyway, I go to MSU, and they offer you a free anti-virus program, Sophos. My brothers also went to MSU and they get Sophos to run perfectly on their computer. But, on my computer, it just doesn't work. I've tried reinstalling it several times and nothing!! This is the message they give me...

You are not a member of one of the Sophos groups. In order to be able to launch this application, you must be a member of Sophos Adminstrator, SophosPowerUser or SophosUser group. Please contact the Adminstrator.

I've tried downloading some free antivirus programs that my windows xp suggest s and they are absolute ****................. ahhhhhhhhhhhhhhhhhhh!!!!!
Help!

 

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