I've seen it before, it means exactly what it says, that it can't grab UDP port 500 cause something else is already bound to it. I presume you've already removed any other VPN clients from this PC, you may find that it didn't uninstall properly though.
Shut down the VPN client service and then open up a command prompt and do a "netstat -an", this'll show you all the ports that are open, check for UDP 500 and see if it's listed. If it is then something is using it and that's why the Cisco VPN client is reporting what it is.
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...