This article shows the steps to take if Microsoft Security Essentials will not install. To do this, you will need to remove any Internet security programs, make sure the Windows installer service is running, and start Windows in selective startup mode.
Remove ALL Internet Security Programs:
1. In Windows XP, click Start, click Run, in the search bar, type "appwiz.cpl" and click Enter.
In Windows 7 type "appwiz.cpl" into the Search box, within the Start menu, and clicking Enter.
2. In the installed programs list, uninstall any third-party Internet security programs.
3. Restart the computer and try the Microsoft Security Essentials installation again.
NOTE: Some Internet security programs may not uninstall completely. To remedy this situation, cleanup utilities may need to be installed and run, for the security application to be removed completely.
NOTE: When removing Internet security programs, your computer is not protected. If Microsoft Security Essentials, still does not install, even after the removal of these security programs, you need to submit a case to Microsoft Security Essentials Support, immediately.
Make Sure the Windows Installer Service is Running:
1. In Windows XP, select Start, click Run, type services.msc and click Enter. Similarly, for Windows 7, type services.msc into the Start search bar and click Enter.
2. Right-click Windows Installer and select Start. If the Start option is not available, the service may already be started.
3. Within the Services window, on the File menu, choose Exit.
4. If running Windows XP: click Start, choose Run, type "cmd" and press Enter.
If running Windows 7: click Start, type Command Prompt and press Enter. Right-click Command Prompt and choose to Run as Administrator.
5. Type MSIEXEC/REGSERVER and click Enter.
6. Try to install Microsoft Security Essentials again.
Start Windows in Selective Startup Mode:
1. In Windows XP, choose Start, click Run, type "msconfig", and press Enter..
In Windows 7, type "msconfig" in the Start search box and press Enter.
2. Click Startup, and uncheck the Load Startup Items check box.
3. On the Services tab, select the Hide All Microsoft Services check box and clear all other boxes for the services that remain.
4. Click OK, and choose to Restart the computer.
5. Now, try to install Microsoft Security Essentials.
Referenced from: Office.microsoft.com