Found some good docs on the Cisco site.
These is my pix interfaces
inside ip: 192.168.1.32 255.255.255.0
outside ip: 194.7.xxx.xxx 255.255.255.xxx
These are the line I added to my pix to create a PPTP VPN possibility
ip local pool pptp-pool 192.168.254.1-192.168.254.254
access-list 101 permit ip 192.168.1.0 255.255.255.0 192.168.254.0 255.255.255.0
nat (inside) 0 access-list 101
nat (inside) 1 0.0.0.0 0.0.0.0 0 0
sysopt connection permit-pptp
vpdn group 1 accept dialin pptp
vpdn group 1 ppp authentication pap
vpdn group 1 ppp authentication chap
vpdn group 1 ppp authentication mschap
vpdn group 1 ppp encryption mppe 40
vpdn group 1 client configuration address local pptp-pool
vpdn group 1 pptp echo 60
vpdn group 1 client authentication local
vpdn username MyUser password MyPassword
vpdn enable outside
A good cisco link :
Have fun !!!!