can't update important files and services while the system is using

1. I get the message "Windows can't update important files and services while the system is using them. Save any open files and restart the computer. and then try to check the new updates." and I am asked to reboot 2. Update history says Definition Update for Windows Defender - KB915597 (Definition 1.139.1946.0) Successful Important Date installed 14/11/2012 (today) 3. After restart: update...