hi Sniggler,
To help ensure updating has as few problems as possible, try this:
Go here to insure no remnants of old uninstalled a/v programs were left behind by a Windows uninstall:
http://singularlabs.com/uninstallers/security-software/ Some vendor tools, such as McAfee or Norton, must be run more than once. Reboot after each run.
To perform a totally clean install of new version of Avast! here: (EDIT: Uninstall Avast! in normal admin mode using Programs and Features first)
Reason for a clean install: Version 6 has older different files and program features than version 7, and updating within the GUI will sometimes result in the sort of problems you had earlier with your XP system. Slow Win7 booting, program hangs, etc., can result.
I know it is more work, but Version 7 is a major upgrade from Version 6, and whilst it is usually OK to upgrade within a version number, best strategy is to remove version 6 completely, reboot, and then install version 7.