Or read the the help.
start > programs > Avast > help
I did actually have a look at help and was offered ...
"Scheduling the Boot Time Scan
Click on the Menu button.
Choose Schedule Boot Time Scan. "
Which is fine if you can get the menu button in the first place (and I still havent found one?) I thought it might have been someting that was not offered on A4H so have learnt it's quicker to ask those who know.
Luckily there are some that realise that we can all need help sometimes .. and that help rarely comes from RTFM or help files (one can't see the wood for the trees etc). If it was the case we wouldn't need support forums so much?
All the best .. ;-)
T i m