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