LinuxQuestions.org - Problem installing vpn client on Ubuntu: Failed to make module "cisco

Hello,
I only use linux occasionally, but I have my laptop split into Windows and Ubuntu. When I tried to install a vpn client on Ubuntu (Jaunty?), I got this rejection:

Making module
make -C /lib/modules/2.6.28-15-generic/build SUBDIRS=/home/bins/vpnclient modules
make[1]: Entering directory `/usr/src/linux-headers-2.6.28-15-generic'
scripts/Makefile.build:46: *** CFLAGS was changed in "/home/bins/vpnclient/Makefile". Fix it to use EXTRA_CFLAGS. Stop.
make[1]: *** [_module_/home/bins/vpnclient] Error 2
make[1]: Leaving directory `/usr/src/linux-headers-2.6.28-15-generic'
make: *** [default] Error 2
Failed to make module "cisco_ipsec.ko".

I installed as many headers with the numbers 2.6.28-15 as I could through synaptec and got the same message. Can anyone help me figure out what to do?
Thanks!