Which operating system, anti-virus software and internet browser are you using?
It's possible your browser is just confused and has an out-of-date ssl cert etc, clearing the browser cache might do it? Or somebody could have reported the site and your anti-virus is just playing it safe? Or it could be a virus trying to redirect you somewhere bad, is the problem affecting any other sites? (Or it could be several other things)
In my experience if a computer gets a virus any anti-virus software is almost useless. It's best to avoid getting viruses in the first place and protecting the browser is the best way to do that. On Windows 7 I use 'Microsoft Security Essentials' rather than a commercial anti-virus and I use Firefox browser with 'Ad-block Plus', 'Noscript', 'Request Policy' and 'Ghostery' plugins. These plugins can be configured to block by default unless you explictly allow websites to run scripts to communicate between each other, this is a very good way to stop advertisers tracking you and to help prevent 'drive-by' virus installs from naughty websites (and it can be as simple as a malicious advert on a normally trustworthy website). (Yes it's a pain the first time you visit your favourite sites to enable scripts and request policy but then those settings are remembered and you'll find you quickly get through and configure the common sites you visit.)
http://windows.microsoft.com/mse
If you're feeling more techie you could modify your computers 'hosts' file which will stop your computer communicating with known virus (and other nasty) distribution channels:
http://someonewhocares.org/hosts/ # It's just a text file that tells your computer how to resolve domain names vs internet IP addresses
I don't use Windows full-time so I'm not really an expert on current threats and fixes etc. Hopefully any windows experts on the forum won't mock me too badly for my advice.
