If avast internet security won't open, you must try to repair the software.
Go to Start
Type 'control panel'
Launch Control Panel
Go to Programs
Uninstall a program
Select Avast
Select Repair.
When the process is completed, you can restart your computer and check if the issue persists.
Still if the above solution doesn't help then you can visit us to learn more about why my avast won't open?