Cant establish a trust thru ISA server

I have two W2K domains, connected via PPTP VPN. I would like to establish a trust relationship between the two domains, but I get an error saying the domain cannot be contacted. I am pretty sure DNS is set up properly, as I can ping the remote DC, but cannot establish the trust. My question is regarding ISA server, and if I have to do anything with that to allow the trust. I have ISA server running on one server, but no the other. Any help would be great.