What previous antivirus software did you have? Run it's uninstall utility in windows safe mode and follow the instructions. After that has been done, do the following below:
a) Uninstall avast premier via add/remove and follow the instructions
b) Download the avast uninstall utility, run it and allow the tool to run your computer automatically in safe mode and follow the instructions.
c) Download and install a fresh copy of Avast premier. It will do a quick start up scan and once it's done, restart your computer manually.
d) Register/enter your licence number of your avast premier
