Sounds like your getting a pop ad from a website. Odds are good that the ad detected nothing and you dont have 79 viruses on your computer. Use a browser with a pop-up blocker and stay away from those bike-porn sites!

If you already have anti-virus software installed, run a complete scan for peace of mind.
If you don't have any, then you should get one, install it, and run a scan.
No need to spend $44.95, or what ever amount they were trying to sell you one for. There are good free ones out there.