Security Essentials Screensaver scans for viruses while your computer is idle
That's why a number of utility apps -- like defrag tools and antivirus programs -- have an option to scan your system whenever it idles. Set your screensaver to the tool's scanner and maintenance is automatically performed while your system awaits your return.
Unfortunately, one of the best free antivirus apps for Windows -- Microsoft Security Essentials -- doesn't have this option. Fortunately, DeviantArt user jwils876 has put together a handy little Security Essentials screensaver download which adds scan-while-idle functionality.
The .ZIP file contains both a graphical and non-graphical version. Copy the .SCR files inside into your \Windows\system32\ folder and then select the version you want to run in Windows' Screen Saver Settings. Whichever you choose to run, Security Essentials Screensaver will force a definition update first and then initiate a scan of your system whenever it kicks in.
[via Into Windows]