Hi,
I'll leave it to the experts to give you a "proper" answer with regard to Avast -- but keep in mind that under XP, DOS is almost non-existent.
A boot-time scan is simple enough even with the "standard" Avast (I use 4-Home). If you open the interface and then its menu, you'll find it about halfway down. "Scheduling" is a bit of a misnomer, in my opinion, since it sets you up for a one-time full scan the next time you reboot, which of course you can probably do right away. In the home version, at least, you'll have to do this again each time you want a fresh boot-scan.
Best,
Mike