The Hosts file can be used to list sites the user doesn't want to connect to. Unfortunately, it is also used by certain viruses/worms to prevent the user from connecting to any site of its choice - including all or most of the sites that have anything to do with virus protection.
looney's suggestion to reboot into Safe Mode is a good one. I've never tried it but it may be that if your computer is infected and your antivirus software won't work in Standard Mode, it will in Safe Mode.
To find the Hosts file, click on Start, go to Find and click on Files and Folders. In the Named field type Hosts, make sure Look in has your primary drive and that Include subfolders is checked, and then click on Find now.
Open the Hosts file in Notepad (making sure always use this program is unchecked). Once you delete the offending entries, save the file and close Notepad. If theres's more than one Hosts file, do the same thing for each of them.