Hi Adam, have not faced this issue yet but have read some arcticles on this error 51 pointing to mostly MAC OS's and not neccesarily on XP, seems installation of vpn client somehow turns on Internet sharing on the system causing this error.. but looking into XP internet sharing you would need at least two NICS to turn on XP internet sharing.. perhaps see if this could be issue on the machine.
the bug ID is not much of a help but if you read the original poster reporting of a another adapter being created on XP this tends me to believe the theory of internet sharing turned on.. well.. at least is worth the try looking into this intenet sharring..
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...