That's the standard warning of the [Microsoft's Windows built-ini] Security Center advise when it didn't detected your antivirus and firewall at boot time.
That thing of course is very strange because the Security System feature (which runs as a service) have been detecting avast! presence from almost 4.5 version and above - forum please correct me if I'm wrong.
I personally don't use this service as it takes ram, system cycles and so on, but the the sympton you describe may be telling -between lines- that avast! is not correctly installed because as I said before the Security Center have been detecting avast! for a while.
May be too that this release of avast! have a bug and that's why SS is not detecting it correctly... please consult with avast! developers or advanced users