I assume you are talking about the boot-time scan that occurs after installation when your reboot and before windows starts ?

If so then this is a one time operation and won’t happen on your next boot, but you can schedule it it required.
If you have XP or Win2k, you could enable a boot time scan. Right click the avast icon, select Start avast! Antivirus, Menu, ‘Schedule boot-time scan…’ Or see http://www.digitalred.com/avast-boot-time.php

If not then the links Tech gave should give you options.