Win 2003 on a Win 2000 Domain

Hello

I have a WIN 2000 domain. I have a remote office which is connected to my
main network VIA VPN. We are using Checkpoint firewall between the sites.
Recently I have installed a WIN 2003 R2 member server in my remote site to
act as a file server. I am having the follwing problems on the win 2003 r2
server.

1. When I go to create a security setting a shared folder, it is not
populating the domain users automatically, even if i click on the advanced
tab, it says the server is not operational.
2. If I type the user and then hit search, it is not able to find.
3. I am able to ping and resolve IP's from my main network back and forth.
4. I am able to termserv from my main network onto the 2003 server without
any issue.

Is there anything specific that I should do for a win2003 server R2 to work
on a win 2000 domain or or to work fine a checkpoint VPN tunnel

thank you