When Windows Live OneCare performs a virus scan, it checks the contents of each file on your computer and looks for the patterns used by known viruses. These patterns are defined in files called virus definitions (A file containing descriptions or templates of the patterns used by known viruses. Also called a virus signature file.) . In order for a virus scan to be effective, the virus definitions must be up-to-date. Windows Live OneCare Antivirus updates your virus definition files automatically when your computer is connected to the Internet. To make sure that you have the most up-to-date version, you can check for updates manually by clicking the Check for updates link on the Windows Live OneCare display.
Although a full virus scan of your computer can find viruses that have already infected it, a virus scan does not prevent new infections from happening. To help protect against that, make sure that Windows Live OneCare Antivirus monitoring is always turned on.