For good install instructions, see my notes about Upgrading to Fedora Core 5.
Any time yum updates the kernel, you need to reinstall ndiswrapper. Just go to ~pjungwir/src/ndiswrapper-1.23 and run "make install." Then run "modprobe ndiswrapper." You should get connectivity without having to reboot.
You also need to reinstall the Cisco vpnclient. That is something like this:
cd ~pjungwir/src/vpnclient
./driver_build.sh /usr/src/kernels/2.6.17-1.2187_FC5-i686/ \" \& is a non-printable, zero-width char
./vpn_install
chmod 4111 /opt/cisco-vpnclient/bin/cvpnd
service vpnclient_init start