Did you completely reinstall avast after upgrading to windows 7 ?
Or was this a clean installation (format) of windows 7 and install of other applications ?
Avast 6.0.1000 has just been released 2 days ago so the registration servers may be very bust. I did a clean install of avast 6.0 on the day of release and it took me many tries to finally get through to the registration process and finally complete the registration.
So it may just be that.