Computer viruses are software programs deliberately designed to interfere with computer operation, to record, corrupt, or delete data, or to infect other computers throughout the Internet. Viruses often slow things down and cause other problems in the process.

Spyware is software that can install itself or run on your computer without getting your consent or providing you with adequate notice or control. Spyware might not display symptoms after it infects your computer, but many malicious or unwanted programs can affect how your computer runs. For example, spyware can monitor your online behavior or collect information about you (including information that can identify you or other sensitive information), change settings on your computer, or cause your computer to run slowly.

Both viruses and spyware are installed on your computer without your knowledge and both have the potential to be intrusive and destructive. They also have the ability to capture information on your computer and damage or delete that information. They both can negatively affect your computer's performance.

The main differences between viruses and spyware is how they behave on your computer. Viruses, like living organisms, want to infect a computer, replicate, and then spread to as many other computers as possible. Spyware, however, is more like a mole—it wants to "move into" your computer and stay there as long as possible, sending valuable information about your computer to an outside source while it is there.

Unwanted software, such as viruses, can be installed by Web sites or by programs that you download or that you install using a CD, DVD, external hard disk, or a device. Spyware is most commonly installed through free software, such as file sharing, screen savers, or search toolbars.

Yes, some malicious software can be installed from a Web site through an embedded script or program in a Web page. Some malicious software requires your help to install it. This software uses Web pop-ups or free software that requires you to accept a downloadable file. However, if you keep Microsoft Windows® up to date and don't reduce your security settings, you can minimize the chances of an infection.

When you visit Web sites, do not automatically agree to download anything the site offers. If you download free software, such as file sharing programs or screen savers, read the license agreement carefully. Look for clauses that say that you must accept advertising and pop-ups from the company, or that the software will send certain information back to the software publisher.

Security Essentials is antimalware software, which means that it's designed to detect and help protect your computer against a wide range of malicious software, including viruses, spyware, and other potentially unwanted software. Windows Defender, which is automatically installed with your Windows operating system, is software that detects and stops spyware. To learn more about Windows Defender, visit the Windows Defender Web site.

Cookies are small text files that Web sites put on your computer to store information about you and your preferences. Websites use cookies to offer you a personalized experience and to gather information about Web site use. Security Essentials doesn't detect cookies, because it doesn't consider them a threat to your privacy or to the security of your computer. Most Internet browser programs allow you to block cookies. For information about blocking cookies in Windows Internet Explorer, see Block or allow cookies.