How can ı enable boot scan?
You don’t say what avast version you are using (7 or 8), it differs in avast 8 ?
That said, generally you shouldn’t need to run a boot-time scan unless there is a problem that can’t be dealt with in normal mode.
Avast 8 UI, Security tab, Scan Now, scroll down to the Boot-time scan.
Or
It’s the free version the OP has ;D