VPN Server connection

i connect to a remote VPN/PPTP Server from my Win XP Pro machine. when i make a connection it fails to connect to VPN Server. but when i assign a Public IP Address to my Desktop & make this connection, It works . when i put back my Private IP Address the Connection fails.

the private ips are behind router with NAT .
i do not have any firewall issue.
whatz causing this behaviour ?