@ 64bit
As far as I'm aware when the registration expires, only the virus definitions can't be updated, the shields should remain on.
This would mean you would have to download the full virus definitions to update the virus definitions. However, this is something which you would already need to do for a system that doesn't have internet connection, otherwise the virus definitions get way out of date.
This could lead a virus infection due to way out of date virus definitions - which inadvertently could disable avast.