I can confirm the delay but am under the impression that it is somehow by intention. I could be wrong, though.

If avast! is running on XPSP2 and configured to have a permanent connection it is trying to update its virus definition data base (or even itself) at startup in an attempt to contribute to SP2’s security enhancements. But, as I said, I might be wrong. avast! also has to communicate its status with the new XP Security center (and this may take some time?).

Perhaps one of the avast! guys could shed some light on this behavior.