Were you previously using an AV or suite that could have left some residue when uninstalled? There are dedicated uninstallers referenced at http://uninstallers.blogspot.com/ for most products. The final version 6 is due shortly so you can try uninstalling your current version using the avast! uninstaller at http://www.avast.com/uninstall-utility and try that. I upgraded to SP1 yesterday on W7x64 and didn’t have a problem with the current v6 beta.