By default, Microsoft Security Essentials runs a scheduled scan on your computer once a week. A weekly scan is sufficient for most computers, because Microsoft Security Essentials monitors your computer continuously through the real-time protection feature. To learn more, see What's real-time protection?.
A scheduled scan checks the areas of your computer that malicious software, including viruses, spyware, and other potentially unwanted software, are most likely to infect. If you want Microsoft Security Essentials to check all files and programs on your computer, you can run or schedule a full scan.
To change the scheduled scan
Click Settings, and then click Scheduled scan.
If the Run a scheduled scan on my computer (recommended) check box is not selected, select it now.
Next to the When field, select the day that you want to run the scan. For example, you can run a scan daily or on a certain day of the week, such as Sunday.
Next to the Around field, select the time that you want the scheduled scan to run.
Note: Scans may begin within two hours of the scheduled time you select. Exact scan times are randomized to reduce strains on network traffic. Scans might also be delayed if something else is currently running on your computer, such as an update.
Next to the Scan type field, select the type of scan that you want to run, and then click Save changes. If you're prompted for an administrator password or confirmation, type the password or provide confirmation.
To learn more about scan types, see Scanning for viruses, spyware, and other potentially unwanted software.