Well, in the end I was able to reproduce the problem on my notebook... which was followed by two days of debugging, trying to find out where the hell the memory gets corrupted (it's not possible to start the boot-time scanner normally from Windows, so I used a special loader to make it possible, which in turn had some strange effects on the keyboard, so I had to control the machine remotely from another computer standing next to it...). In the end I found something wrong in the handling of threads in the boot-time scanner (well, "wrong" means it seemed to be wrong... the whole thing is undocumented and I didn't want to trace deep inside of Windows kernel, because Vlk - who knows more than me about NT kernel - is skiing somewhere
)
Anyway, if you're interested, you can try the updated version
here.
Just note that when you overwrite the original file in Windows\System32 folder with this new one, avast! setup will notice soon and overwrite it back with the official build - so you'd better overwrite it right before the restart (after scheduling the boot-time scan).