What version of avast are you using? Version 8 or 2014?

To disable it at the avast icon on the bottom right and corner, right click on the avast icon, go to avast shields controls, and click on disable permanently.

Another thing that you can do is this! Before doing the above. Download and install SuperAntiSpyware, MalwareBytes Antimalware, and HitMan Pro and update them. Run windows in safe mode and run SuperAntiSpyware, MalwareBytes AntiMalware, and HitMan Pro. If this doesn’t help do the one that I mentioned above.