Hi! I am having the same problem, although I have been using avast 5, and before it 4, for years instead of days. I think it has to do with the fact that I am running Windows XP SP1 on this system. I can not upgrade this computer to SP2 or higher because I have some older software, games, that can not run using Windows Media Player 9 or higher. They require version 8. I did not check the web site, which states that it only supports Windows XP SP2 or higher, until after I had allowed Avast! 5 to upgrade and found that windows could not start. I am surprised that Avast! 5 did not notify me of this new system requirement or warn me that my system did not meet the minimum requirements for Avast! 6 before it performed the upgrade. I have been looking around for the last version of Avast! 5 so that I can download and reinstall it on this system, but have not found it as yet. Hopefully a link can, and will, be provided.