Microsoft Security Essentials Exclusion List

To add exclusions, click on Settings > Excluded files & locations, then click the Add button to add an entry.

For Windows XP, add the following exclusion:
C:\Program Files\Microsoft Security Essentials

For Vista/Windows 7, add the following exclusions:
C:\Program Files\Microsoft Security Essentials
C:\Program Data\Microsoft\Microsoft Security Essentials
C:\Program Data\Microsoft\Microsoft Antimalware