Microsoft Security Essentials (MSE) is the light-weight, one of the best and free antivirus software from Microsoft for Windows 7, Vista and XP.
If you have installed MSE on Your PC and incase face any issue with it, here is a freeware tool called Fix MSE (unofficial) to repair it with out reinstalling Microsoft Security Essentials.
Simply run Fix MSE utility. It will restore all its registry values & service settings to known good defaults, without re-installing MSE.
The utility may take only a moment to run, and works on both 32bit and 64bit of Windows Operating Systems.
Requirements: Microsoft .Net Framework 2.0, Microsoft Security Essentials 1.0, Windows 7, Vista and XP. Download Fix MSE