There may be some misunderstanding here (from my side as well)…

If you refer to the boot-time scan, it’s not about scanning the boot files - it’s just a scan performed during the boot, i.e. before the operating systems has started (and before any Win32 virus has had a chance to run). It is available on Windows NT/2000/XP/2003 only. However, the scan target is whatever you select - all your hard disks or selected directory.

If you are asking about scanning “the group of files that may be automatically started when you start your computer”, avast! can’t do it (yet - but we were thinking of it some time ago already, so it may appear in the next version…).