What was the file name of the attachment ?
I would suggest that you contact sales (at) avast (dot) com, with details about the date purchased, email address used and the problem of not being able to insert from the avastUI, Registration.
Have (or did) you another Anti-Virus installed in this system, if so what was it and how did you get rid of it ?
As that can have an impact on the avast installation and also possibly cause conflict on boot.
If you can boot into safe mode that should allow you to uninstall avast removing any conflict and hopefully allow a normal boot into windows. to try and correct any conflict.