Wow, this thread is so familiar to me. I was having the same exact issue on my laptop after upgrading to Avast 6 Free. I would get occasional BSOD crashes and atapi.sys was always listed as the cause. I initially thought it was hardware related too. Ran every diagnostic I could and nothing found any errors - memtest, seatools, etc. Then, one day
I tried to run a quick scan with Avast and it BSOD'd instantly as soon as the scan started (atapi.sys again). Did that 3 times in a row. That is what made me think the new version of Avast was causing the problem so I uninstalled it and replaced it with MSE. After that, no more crashes. This was after I had problems on my desktop with Avast 6 Free too. Not BSOD, but it was hanging whenever I ran a scan with it. I had several threads on this a few weeks ago and tried everything suggested but the problem persisted so I put MSE on that machine too. My 3rd PC is still Running Avast 5 and that one has no problems so all of this leads me to think something is quirky in version 6. I'd prefer to return to Avast in favor of MSE but I'm going to hold off until it looks like whatever was causing the issues has been found and corrected. I've been using Avast since 2005 (I think) and have always been a fan so I hope to return to it.