When you say it crashes is it only when you kick the app up?.. I would try un-installing the vpn client with all folders and files out of the C:/program/Cisco systems/vpn client and re-install a downloaded clean copy. Then when the pc boots into windows install fresh install of cisco vpn client and then from the start/run and type msconfig and de-tick any files that don't need to start straight away including vpn client and then reload(as the vpn service doesn't need to run all the time), and at the next login prompt you'll be made aware of changes to startup, tick the box(don't show again) and click o.k, I'm pretty sure you need the determininstic network enhancer for the vpn client to work but kick the vpn up and then if still crashed goto the event viewer in administrative tools and see if the error is seen by the OS..Let me know if this work:).
