A Windows 2000 machine could access our VPN at work and use Remote Desktop Client to log into any of our servers. A Mac could access our VPN at work and use Remote Desktop Client to log into some of our servers but not all of them.
Initially I thought it might be a subnet issue but later I discovered that I could access all of the machines on one subnet but only some of the machines on the other. My coworker spent a significant amount of time trying to figure out if there was any difference between the machines that would explain why I could only access some of the servers. As far as he could tell the machines were virtually identical.
Yesterday I started poking around in the Computer Management utility trying to see if I could find any differences between the machines. After some false starts I realized that all of the machines that didn't work were on the same subnet and had the same version of wsock32.dll. Other machines on the same subnet that did work had different versions of wsock32.dll.
Our network guy came in today and determined that Windows 2000 Service Pack 1 and Windows 2000 Service Pack 3 appeared to be working and Windows 2000 Service Pack 2 on one of our subnets was not working. He then applied Service Pack 3 to one of the problem servers and now I can connect to it through our VPN.
So, if you're attempting to use MacOS X with a PC VPN you may run into problems connecting to Windows 2000 Service Pack 2.