Mine keeps doing the same for .NET 3.5 sp1.
I've repaired it, reinstalled it and still the same.
Next time it returns, uncheck it, & check "Do Not Ask Me About this Update Again".
Update icon returned again. Clicked it, icon disappeared.
MSE won't update either, even after full uninstall & then reinstall.
"...definitions didn't update because of an Internet or network connectivity problem."Log says something different:
Microsoft Security Essentials ( Success Software Synchronization Windows Update Client successfully detected 1 updates.
then:
Microsoft Security Essentials ( Failure Content Download Error: Download failed.
Probably something Microsoft needs to fix.Maybe Step #4 is next to try:
http://windows.microsoft.com/en-us/windows/troubleshooting-update-issuesTried #4, no change.