Here is some good quick advice from my fellow blogger Debra Radcliff:
Panda Security reports increased spread and success of popup “security warnings.†These warnings popup when people surf the Web and hit a malicious or infected Website, and keep flashing their warnings until the user goes to the link, at which time they get infected.
No legitimate security company would do this to a computer, so don’t click the link. Instead, disconnect from the Internet, clear your browser history and restart your computer. If your browser is still flashing warnings, the system will need to be disinfected through anti-virus or a computer restoration service.
Usually these false security warnings are a symptom of something much worse. I’ve had some that will actually not allow you to do much of anything but click on the link in their fake pop-up. What I did was a system restore, but you can also boot in Safe mode and attempt to clean the system.