If you are unable to access any Anti Virus sites on the net from your PC , then W32.Downadup worm may be the reason.
This worm infects computers Hosts file and also prevents your existing anti virus software from downloading and installing updates.
To remove this worm follow the steps below:
1. Open command line window – Go to Start > Search , type in cmd to open command prompt window. For windows XP users, go to Start>Run and then type cmd
2. Type net stop dnscache – this will stop the DNS cache service so that you can access and download tool to remove infection.
3. Go to: http://www.symantec.com/security_response/writeup.jsp?docid=2009-011316-0247-99 – download W32.Downadup tool.
4. Run the tool. When finished reboot your machine.
5. Check out if the infection is removed by restarting the DNS cache service ( net start dnscache) and visiting any Anti Virus site like AVG, Symantec etc.