I had the same problem yesterday when upgrading a 10Mbps network card to a 100Mbps card.
I uninstalled the 10Mbps card, restarted Win XP, after installing the 100Mbps card, and McAfee firewall threw up a requester saying something about windows socket being changed/upgraded and wham, IE6, Oulook Express, MailWasher etc. etc. etc. all stopped working.
After browsing help, without any luck, I did a system restore and restarted, I had to reinstall the drivers for the 100Mbps card and again the firewall presented the same requester on restarting but this time all was OK.
Any of this any use to you?