Hey..
I am also Dell M6400 user and have a great solution for you.
For a repair of avast. Windows, Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button from the pop-up window, scroll down to Repair, click next and follow.
I HOPE this might work for you. If not then you can ask experts for help.
For more information and support visit Dell Precision M6400 Manual- guideusermanual.com/product-name-precision-m6400-manual&po=344511&lang=English. Good Luck.