Sad but true. While these issues are not consistent for all they are widespread for many. It seems to be optimized for Win7. Looks good and works well with some nice innovations. Win7 issues get fixed quickly. Vista and XP (especially XP) are not so fortunate. Many people are reporting Blue Screen crashes frequently with Avast 5, mostly on XP systems. I'm one of them. After an upgrade install it immediately crashes when the desktop loads. Safe Mode>Uninstall>Reboot and it's fine. Run the Avast Cleanup tool and reinstall V5 fresh. Blue Screens when desktop loads. Avast is suspecting something to do with a PowerDVD files, but there are reports from people that have never had PowerDVD having the same issues. I'm back to 4.8 which still works just fine. I'd recommend you download a copy of 4.8 and the Avast cleanup tool before attempting to upgrade to 5, or better yet, just don't upgrade to 5. Wait until a new build is out that hopefully fixes these issues. After the recent bad definition update that started deleting system files, this comes as very unwelcome news. I'm losing faith in Avast. Hopefully this will be resolved quickly. It's seems a bit silly to optimize it primarily for Win7 when Vista is bound to be around for a good while yet and even XP will be here for the long haul. XP is still the most widely used OS today. These problems exist as of build 5.0.377. Wait until the next build is out and check their forums for success reports before trying it.