I assume you're running Windows... if so, check your hosts file... many virus like to rewrite that file. Basically, it is a list of web sites with their IP addresses, so no DNS server is needed to reach them. If a virus rewrites the file with entries for common web sites like Google, MSN, Yahoo, etc., you could type in a legitimate URL in your browser and end up at a fake site, or a drive-by download page, etc.
Do a search for "hosts" without the quotes (and no extension) and once you find it, open it up and take a look; normally it will only have one or two entries. Be aware that some anti-virus/anti-malware programs (like Spybot S&D) will add a ton of entries to the hosts file... they add entries for known bad web sites (virus, malware, spyware, etc.) and redirect them so you can never accidentally go to those sites, so if you have a bunch of entries, it may or may not be the issue.
Proceed at your own risk...