XP to Win2k VPN problem

I am running Win2K Small Business Server. I have a VPN set up through RRAS, and I can connect to it just fine using Win2k Pro cleints.
We got a new laptop which has WinXP Pro. I’m going through the same Networking/Dial-Up process as I did with Win2k Pro, but the VPN connection will not go through. It hits the IP address and then gets to “Verifying User Name and Password….” sits there, and eventually quits with an “Error 721: Remote system is not responding.” message.
I can use the same username and password on a Win2k VPN setup/machine and it will connect just fine, it’s only the XP Pro machine that has trouble. I know the users are allowed Dial-In access… I’ve checked everything I know to check, and I’m stumped.