1. The original question was whether avast protects against rootkit, and the answer here is: "of course". My estimate is that nowadays, 30+ percent of malware actually comes with some kind of rootkit (to protect/hide its files), and if we weren't able to block rootkits, we'd be missing a huge number of malware.
2. Another question is if avast is able to detect (and more importantly, remove) active rootkits if installed on an already compromised machine. Now, the answer is: "in many cases no", and that's indeed where specialized anti-rootkit tools do a better job. That said, I can say now that we're coming up with a new rootkit detection/removal utility shortly.
3. Someone said that most AV programs already contain some rootkit detection/removal tool. However, have you actually measured the success ratio of these tools? We have, and the results were quite amazing. Only a handful of the tools is actually doing a decent job. The morale of the story is that if someone says a product "can deal with rootkits" doesn't actually mean that it's doing a good job.
4. While "layered defense" is a good concept, it doesn't quite work here. The frontiers betwen Trojans, Backdoors, Worms and other types of malware are now so vague that there's no specialized anti-Trojan, anti-Backdoor or anti-Worm tools anymore. Anti-rootkit tools are a bit different (as they are "heuristic" in their nature, i.e. they search for hidden items, without any need of defitions/signatures), but still, the technology also belongs to AV programs.
Hope this helps,
Vlk