While you are upgrading the Microsoft Security Essentials you may encounter an error that you can’t update or upgrade your Microsoft Security Essentials. Microsoft Security Essentials use Microsoft Update (MU) service to update and you must be careful to check that whether your Microsoft Update service is properly working or not. If something happened and your Microsoft Security essentials does not upgrade or update, you should check the Microsoft Update service in the service panel. If there is no Microsoft Update service in the services panel you must activate the Windows update by going to the properties of My Computer and then to Automatic Updates and If you Windows Updates are activated but still you can’t update and upgrade you must restart the service.
Restart the Microsoft Update (MU) service
In Windows XP, click Start, click Run, type services.msc, and then press ENTER.
Right click Automatic Updates and then click Start. If Start is unavailable, click Restart.
In Windows Vista and Windows 7, click Start, and in the Start Search box, type services.msc, and then press ENTER.
Right-click Windows Update, and then click Start. If Start is unavailable, click Restart.