Sergio,
I have just tested the following configuration on my lab PIX 515. I connected to the PIX using a dialup to the ineternet from my laptop and setup a PPTP connection from it to the PIX and it works with no problem.
access-list pptp permit ip 10.x.x.x 255.255.255.0 172.x.x.x 255.255.255.128
ip local pool pptp_dial_in 172.x.x.1-172.x.x.10
nat (inside) 0 access-list pptp
sysopt connection permit-pptp
vpdn group PPTP-VPDN-GROUP accept dialin pptp
vpdn group PPTP-VPDN-GROUP ppp authentication chap
vpdn group PPTP-VPDN-GROUP ppp authentication mschap
vpdn group PPTP-VPDN-GROUP ppp encryption mppe auto
vpdn group PPTP-VPDN-GROUP client configuration address local pptp_dial_in
vpdn group PPTP-VPDN-GROUP client configuration dns
vpdn group PPTP-VPDN-GROUP client configuration wins
vpdn group PPTP-VPDN-GROUP pptp echo 60
vpdn group PPTP-VPDN-GROUP client authentication local
vpdn username password
vpdn enable outside
I hope this helps.
Jay