Eddy
6
I hope you ran CCleaner a couple of times as I suggested.
Click “registry”
Click “scan”
If something is found, click “remove”
Repeat those 3 steps till nothing is found anymore and reboot.
Before trying to install avast again, make sure that a pre(vious) installed av is removed completely first.
Links to removal tools and instructions: http://www.ache.nl