There is a huge attack going on right now, affecting hundreds of thousands of websites. When you go to an infected website, you get redirected (I believe via a pop-up window) to a malicious site that tells you that you have malware on your computer and tells you that you can clean it by installing the free antivirus software called Windows Stability Center. If you click on that link, you will infect your system and eventually ask you to purchase other antivirus software.
NEVER click on a link that pops up like this. If you think you might have a virus, close that window and run you well-trusted antivirus software. (Please see my “Essential Security Measures for Home Computers” document if you need help with choosing and antivirus solution. If nothing else, download the free version of Malwarebytes )