For me it sound like if your computer is part of a domain. (Server enviroment)
If so, you should contact the administrator.
And yes, the antivirus will work even on a guest account.
If you are not sure if your computer is on a domain (virus or administratively done) do this:
Start -> Right-Click "My Computer" -> Properties -> Computername (Tab)
Look for something like: "Workgroup: MSHOME", or "Domain: (Something)"
If it's a workgroup machine, your account is probably damaged, and this can be bad.
Best thing to do in this case, would be to go to "My Computer" -> Windowsdrive (Standard C: ) -> Documents and Settings -> (Your Username)
And backup everything (burn it to a CD/DVD or place at a external drive).
Make sure when you copy, that hidden files where on, so you won't have any missing files.
Then do:
Start -> Right-Click "My Computer" -> Properties -> Advanced -> Userprofiles Settings (middle)
Select your user from the list, and choose delete.
WARNING!, THIS WILL DESTROY ALL YOUR USERDATA, SO MAKE SURE YOU'VE BACKED IT UP!
Restart your computer, and it should login fine again, if so, copy your backup into the user-folder again.
Be aware that your problem could be recreated while doing this. If so, write here again.
And please post which internet browser you are using, so I can guide you how to re-copy only the important files to your fresh user.