Well, the fixed IP address turned out to be the relevant clue. I changed this back to automatic assignment from the router's DHCP service, rebooted, and had no connection. Investigating this showed the XP DHCP client service had somehow gotten disabled. Resetting this to Automatic solved the issue. Apparently Comodo likes to see the DHCP handshake happening for whatever reason, even if it does not actually need the IP address - I can even go back to fixed IP address now without getting that 90 minute delay any more!