Hi Leo,
Is there any other security software installed on your computer other than Security Essentials?
I would suggest you to check the update history and see if there are any updates related to Security.
See which Windows updates are installed
I would recommend you to uninstall and reinstall Security Essentials and check if it helps.
Notes:
Once you uninstall, refer to the Step 3: Reinstall Microsoft Security Essentialsfrom the above Microsoft article.
****The steps mentioned below include system registry modification. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base: Back up the registry: http://windows.microsoft.com/en-US/windows7/Back-up-the-registry
Antivirus software can help protect your computer against viruses and other security threats. In most cases, you shouldn't disable your antivirus software. If you have to temporarily disable it to install other software, you should re-enable it as soon as you're done. If you're connected to the Internet or a network while your antivirus software is disabled, your computer is vulnerable to attacks.