I PMed Essexboy for you and he should be able to use FRST to blow away Avast but you need to run the scanner first and post/attach the log.
Take a look at this thread (yes, I know it is a Virus removal thread) but go down to the “If you cannot Boot the computer” and follow those instructions and post the log from FarBar.

Also, what O/S are you running ?
It sounds like you can boot into Windows to you can always do a in-place upgrade which will re-install O/S and not effect your other programs. Hopefully, Essexboy’s help will mean you don’t need to do this.

Also, I’d go ahead and make sure you have all your important files backed up to USB HDD.