Use this patch if you want to be able to use
a windows machine to vpn out through the linux firewall
to a pptp server on the internet.

PPTP-Client-->LinuxFirewall-->Internet-->PPTP-Server

to install:
download to /usr/src/
cd /usr/src/linux-2.4
patch -p1 <../netfilter-pptp-2.4.18.patch

make menuconfig
goto network options->netfilter
and set conntrack_pptp to <M>
you can turn debug on if you want to <*>

exit and save

make modules SUBDIRS=net/ipv4/netfilter
make modules_install
restart server

NOTE: If you apply this patch before running pptpsetup.sh
from the other instructions on this site, you can check
the appropriate box when it runs make menuconfig and everything
will install fine. You won't have to run the make modules 
instructions in this help file.