I have WinXP, and Avast 8 gave me nothing but trouble. Boot up and shutdown were WAY slower, and I posted in here that I could not install a new program without a major battle that Avast won by simply stopping the install progress or lagging on for years just spinning in systray.
I went back to version 7, MUCH better. I kept getting a pop up that another "version 7" upgrade was available, thought about it, and finally upgraded to this other newer version 7.whatever. It turned out to be version 8, and within 15 minutes there was a blue screen crash - followed closely by the removal of version 8 (again), replaced by that other version 7 I had in my download directory.
Don't get me wrong ... I like avast - but this version 8 has a problem it seems with Win XP I'm guessing. I don't see myself getting rid of XP anytime soon, because, quite frankly, everything after XP was a piece of crap. My son got a new laptop, gave me his old one with Vista ... THAT was replaced by XP as well LOL. See where I'm going with this