I read the quote below from the boot-time scan section in the avast! user interface.
The boot-time scan is one of the main weapons of avast! It runs before Windows is loaded, and before potential malware has a chance to activate, thereby enabling an infected system to be cleaned before any damage is done. Moreover, the avast! boot-time scan uses direct hard drive access, which bypasses the OS file system drivers and is thus able to handle even the most stubborn rootkits.
I don’t really know about rootkit, do you think avast! can act as a substitute for all the rootkit scanners out there, e.g. GMER? That is to say, is it ok to only use avast! as the only rootkit scanner and forget about the other rootkit scanners?
Lets put it this way, why would you need all the rootkit scanners ?
These are generally specific tools if you suspect a rootkit is present, then you might run one.
The avast boot-time scan is also a specific purpose tool, e.g. you detect something in normal mode that can’t be dealt with in normal mode.
Avast already has an anti-rootkit scanner built into the program (based on GMER, who now works with avast) and is run 8 minutes after boot. The anti-rootkit is also run as a part of the Quick and Full System Scans, this is at a slightly different level of sensitivity depending on the scan run.