Guessing a little here, but it sounds like that UAC might be blocking it if you have only just turned it on.
You might have to allow avast to run as an administrator, or you might need to re-install Avast with UAC turned on, so that UAC lets Avast install with the correct permissions.
I'm running Avast free on both Vista-32HP and Win7-64HP, and the stats are working on both.
regards, Gizbar.