Run Msconfig

1.In the System Configuration Utility dialog box, click Selective Startup on the General tab.

https://dl.dropboxusercontent.com/u/73555776/Cleanboot1.JPG

2.Click to clear the Load Startup Items check box.
NoteThe Use Original Boot.ini check box is unavailable.
3.Click the Services tab.
4.Click to select the Hide All Microsoft Services check box.

https://dl.dropboxusercontent.com/u/73555776/cleanboot2.JPG

5.Click Disable All, and then click OK.
6.When you are prompted, click Restart.

Now reboot several times to see if Avast stays on