How can I tell if my computer is infected with malicious software?

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

You notice new toolbars, links, or favorites that you did not intentionally add to your Web browser.
Your home page, mouse pointer, or search program changes unexpectedly.
You type the address for a specific site, such as a search engine, but you are taken to a different Web site without notice.
Files are automatically deleted from your computer.
Your computer is used to attack other computers.
You see pop-up ads, even if you're not on the Internet.
Your computer suddenly starts running more slowly than it usually does. Not all computer performance problems are caused by malicious software, but malicious software, especially spyware, can cause a noticeable change.

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?