There are indeed viruses that prevent Antivirus software from updating and running properly they even block acces to all kinds of antivirus sites.
You'll end up with a computer that is shouting out to the internet I AM HERE, NO PROTECTION
I ran into that once on a friends comp.
It would just barely start up and then it was just busy.
You could hardly do anything with it.
Browsing was impossible, it would just trigger popups.
It was the worst case I ever ran into
Loaded with Spyware and viruses, they were competing for the CPU
In the end I managed to clean it, I like a bit of a challenge but a reinstall would have been lots easier
But anyway here's my first aid List:
See if you can update your virusscanner and do a full systemscan.
If you haven't got a good one try this good freebee:
http://www.avast.com/eng/avast_4_home.html
Download and run Adaware:
http://www.lavasoftusa.com/support/download/
And Spybot S&D:
http://www.safer-networking.org/en/download/index.html
Make sure both apps are updated before you scan.
And here you can have your comp scanned for trojans:
http://www.windowsecurity.com/trojanscan/
Or even better , Download and install the Cleaner (Buy or use 30 days free)
http://www.moosoft.com/products/cleaner/download/
And I think it is wise to go to a specialised forum where you can post a Hijackthis log.
http://www.geekstogo.com/forum/index.php?showtopic=2852
And here's some more info:
http://pcpitstop.ibforums.com/index.php?showtopic=61975
Good luck