I'd like to add my experience to this thread.
Avast 6.0.1125 free was installed on my Win XPPro SP2 system. Same days ago it started to crash few minutes after boot (it didn't missed one) and the popup alerted me with a rootkit problem with system32/drivers files. Not the same files every time, more frequently nklnk*.sys.
An Avast complete scan revealed nothing. McAfee Rootkit Detective, nothing again. Rootkit Revealer was not able to access registry hive.
A couple of weeks ago I had problems with Skype (last updated version) as well, it started to crash just after launching. I was forced to disinstall it.
I made a clean disinstallation of Avast 6.01, reinstalled from scratch, problem again. Disinstalled again and scaled back to Avast 5.1.889 and everything works fine now, Skype (reinstalled) works fine again, Rootkit Revealer is working properly now (though nothing suspected has been found).