A Russian support forum has a lengthy thread with users posting about an issue with an Avast! Antivirus update deleting a critical system file on their Windows XP machine, tcpip.sys.
A livejournal post from what one of the forum posts describes as an Avast developer confirmed the issue in the December 5th update and offered a method for repairing the issue. Since the update breaks network connectivity, any affected user will have to seek the solution out from another machine (that they hopefully haven't also installed Avast! on).