Cisco VPN client most often requires to be compiled for the linux header you are running. And with the subtle changes in the header versions, its likely that you would run into this error while installing the vpn client:
/usr/src/vpnclient/GenDefs.h:113: error: conflicting types for 'uintptr_t' include/linux/types.h:40: error: previous declaration of 'uintptr_t' was here
In case you didnt notice this error while installing, you will surely notice this error when you try to start the VPN daemon:
'/lib/modules/2.6.24-19-generic/CiscoVPN/cisco_ipsec.ko': -1 Invalid module format
Like HHGG says, dont panic. The VPN client should be patched with the delta from http://projects.tuxx-home.at/ciscovpn/patches that corresponds to your linux header. Patch your vpnclient as mentioned below and you are good to install!
cd vpnclientfolder wget http://projects.tuxx-home.at/ciscovpn/patches/vpnclient-linux-2.6.24-final.diff patch < ./vpnclient-linux-2.6.24-final.diff
Its not that theres a dearth of information on the web on this topic, I still end up wasting time everytime I install vpn. So here I go underlining whats been already said.