I think its like lots of forums and there is a moderation delay for new posters, think we know all about that procedure! Jo

Personally, I would try disabling avast temporarily while installing the NBI
Right click on the avast symbol in the bottom right hand corner of the screen
Click on Avast control shield and select disable until computer is restarted.
you will get a warning window, click yes
Avast will turn off, and a little cross will appear over the Avast icon
Install the NBI and restart the pc or ...
left click the Avast icon and click FIX in the Avast window which tells you Avast isn't working
Jenny
ps... if Avast refuses to turn off then you need to check if your pc man has given you administrator rights.
go to control panel> user accounts and see what it says against you as a user. Personal PCs usually say administrator under your username icon, which means you have freedom to do what you want with the pc.
If it says standard user then you are limited to using the existing programs and some settings that do not affect other users of the pc.