Make sure you have the following command in your config:
sysopt connection permit-pptp
This will allow PPTP packets in to bypass all ACL's and static's etc.
If you have that already, make sure you're passing down your internal WINS and DNS server to the PPTP client with the following:
vpdn group 1 client configuration dns 10.1.1.1
vpdn group 1 client configuration wins 10.1.1.2