First:
1.) Make sure you are registered and your license key is current.
2.) Open the program and click Update Program, following any prompts that may appear.
If these two steps don't work:
3.) Go to Control Panel and find Avast Under Add/Remove Programs. When you click "Remove" there will be a Repair Diagnostic Tool. This will repair any missing, damaged, or outdated files. After following the prompts and it is finished, wait about 30 seconds, close all other Windows and reboot your computer.
Sometimes the Windows Security Center gets confused and the Repair Option fixes problems like this.
Jack