A Little help for a php n00b please?

[Redhawk]

Recently we have been having issues with certain members continuously registering and posting abusive and threatening messages, including and I quote "if i where you , i would be leaving your home sorry to tell you a lot
off bad ass cheaters out there who hate asses like you enough to kill
.... your pants little wet now? lmfao , you deserve every single bit off
it for making such a retarted forum".

We took the liberty of reporting this to the users' ISP, along with logged IP information. However the reply we got was that the ip address we had was assigned to one of their webcache servers. They were kind enough to provide us with information about changing the coding so as to show the real IP address, they advised that "This script is looking at the wrong
variable. REMOTE_ADDR and REMOTE_HOST will both respond with the Inktomi (Cacheing)
traffic servers. However, if you use HTTP_CLIENT_IP or
HTTP_X_FORWARDED_FOR this will report the correct IP address of the
user.

Which file would I need to edit to change this variable so we can progress with this complaint?

Thanks in Advance.