PORT Changing

Hi guys,

I've got my XP PC connecting to the internet via cable,
which is shared between the PC itself and my other home
network, via a ethernet board.
The thing is, I have an application running in my PC's
(which connect the internet via my home ehternet LAN to
my main PC, that connects to the internet) that have to
use a specific port that cannot be changed.
My main PC that shares the internet connection, somehow
changes the port in which that application connection is
made. The application starts on port 500, enters in the
ethernet in my main PC as source and destination port
500, but in my internet card output the source port now
is 1025 (though dest port stays 500).
Is there a way to avoid this, that is, keep the same src
and dest port in my "router" PC?