If you suspect that you have a virus or spyware, run a full system scan. A full scan will slow your PC down while it is running, but that’s because it’s checking every nook and cranny of your computer.
By default, Microsoft Security Essentials runs regular scans. But you may want to start a new scan if any of the following things happen:
You notice changes that you didn’t make—new toolbars in your web browser, a new home page, or a new mouse pointer.
You type the address for a specific site, such as a search engine, but you're taken to a different website without notice.
You see pop-up ads, even if you're not on the Internet.
Your PC suddenly starts running more slowly than it should. Not all computer performance problems are caused by malicious software, but malicious software, especially spyware, can cause a noticeable change.
To scan all areas of your PC (Full scan)
On the Microsoft Security Essentials Home page, select the Full scan option, and then click Scan now. The scan may take a while, depending on the number of files and folders being scanned.
To scan the areas of your PC that malicious software is most likely to infect (Quick scan)
On the Microsoft Security Essentials Home page, click the Quick scan option, and then click Scan now. The amount of time the scan takes depends on the number of files and folders being scanned.
To scan a specific file or folder (right-click scan)
If you suspect malicious software has infected a file or folder on your PC, or if you're concerned about something that you downloaded, you can select a specific file or folder on your computer for Microsoft Security Essentials to scan.
Right-click the file or folder on your PC, and then click Scan with Security Essentials.
Microsoft Security Essentials will begin scanning the selected file or folder.
As soon as it completes the scan, Microsoft Security Essentials displays the scan results. Depending on the file size, this scan may take a few seconds.