According to the Microsoft Malware Protection Center, there is a new malware attack that mimics the look and feel of Microsoft Security Essentials, the free and (I think) very good antivirus software from Microsoft.
The malware is found online, and is able to detect what web browser (Safari, Firefox, Chrome, Internet Explorer) you are using at the time. It makes you think that the site you are visiting has possibly infected your computer, or may be a “phishing” site. It has a link to click to download an antivirus program to scan and fix the problems. This program is called “Win7 AV” and looks a lot like a typical antivirus program. It will install, and run a “scan” that will tell you to go online to pay for a fix.
You can tell by reading the site carefully that it is a hoax. For one thing, some words are misspelled. For another, it links directly to a piece of software to download and install. A real alert will not automatically link you to a fix, but will tell you to take precautions. The software that tells you to purchase a fix, that’s also bogus. Careful browsing will avoid traps like this and others.