Straight from the Avast help file:

Schedule Boot-Time Scan. Displays a dialog allowing you to schedule virus scanning before the operating system start. You can remove the virus before its activation this way, because almost all viruses start along with the operating system. During this test, avast! creates a report file named aswboot.txt in C:\Program Files\ALWIL Software\Avast4\Data\Report. This function is available on Windows NT, 2000 and XP only.