You might have some form of malicious software, including viruses, spyware, or other potentially unwanted software, on your computer if:

There might be malicious software on your computer even if you don't see any symptoms. This type of software can collect information about you and your computer without your knowledge or consent. To help protect your privacy and your computer, we recommend that you run Microsoft Security Essentials at all times. For information about removing malicious software, see What should I do if Microsoft Security Essentials detects malicious software on my computer?