Unfortunately the XP firewall has zero outbound protection, so it shouldn't interfere with the avast updates.
Is the program showing that the License is a) valid, not invalid or b) the license has expired ?
If either of these are correct then avast wouldn't let you update.
So when you bought the license was it for avast Pro version 5.0 ?
If so was that entered in the UI, Maintenance, Registration section ?