Totally depends on your OS which you haven't mentioned.
If you have and NT based OS, such as XP, then you can schedule it from within avast, also see avast help file - avast! - help: simple user interface 'How To'
Well, no idea what NT and OS means, but no such thing as "schedule boot scan" icon appears when right clicking it. And I get the "A Virus Was Found!" message often, it complains about the trojan.gen virus I saw here, on the forums, and about the Win32:Korgo-V [Wrm] worm, and I really have to boot scan, to remove them, Im scared actualy, got no way to backup, got no time either, Im attending a LAN, and messages about exploits from LSASS from some number adress appears, also the DCOM exploit, I took a screenshot of the message, the number is 10.0.1.137:135/tcp (for the DCOM), please help me!