An antivirus program works in two ways: it helps protect your computer from becoming infected, and it "cures" your computer if you've been unlucky enough to be infected.

Microsoft offers three antivirus products you can use that provide you with powerful antivirus protection and recovery:

• Microsoft Security Essentials is our fully featured, real-time antivirus solution for your home or small business. It runs quietly and efficiently in the background, with no annoying pop-ups.
  It can be left on at all times, and can automatically update itself to the latest set of definitions, so you're always protected.
• Microsoft Safety Scanner is recommended if you aren't sure if your antivirus program is working properly. It's designed to be used as a temporary measure that gives an extra boost to your antivirus protection.
  If you think you're infected with malware, but your antivirus program isn't detecting anything, you could use this to get a second opinion.
• Windows Defender Offline is a powerful offline tool you can use if you suspect you're infected by certain advanced malware, such as a rootkit. It's a tool that you can create on a clean computer and use on the computer you suspect is infected.
  It contains a fully up-to-date set of definitions, and performs a rigorous, in-depth scan and recovery of your computer.