Hi Fawad,
For RDP to an internal host from public network require:
1. Static NAT which opens port for communication to internal host on a Public IP.
2.Access-list entry permitting traffic to real IP of the server.
For details refer following link:
http://www.cisco.com/c/en/us/support/docs/security/pix-500-series-security-appliances/77869-pix-remote-desktop-conn.html
Thanks
Rishabh