I have a very small customer that has a single server running Windows 2000 Server as a DC and DNS. All the PC's are currently running XP but they want to replace one of them with a new Windows 7 PC. Will I have trouble joining the Windows 7 PC to the Windows 2000 Domain?
Yes, you can join newer PCs to older domains, and older PCs to newer domains. Just keep in mind that you won't be able to benefit from all of the functionality that a 2008 R2 / Win 7 Professional setup will provide.
But if you're just using the domain for basic SSO stuff, and Exchange integration, then you'll be fine.
We have joined several windows 7 professions workstations to an old windows 2000 domain without any issues.
Joining a domain shouldn't be a problem, but please beware of LmCompatibilityLevel when trying to access resources via the network on the Windows 7 machine, by default Windows 7 uses NTLMv2 earlier systems dont use it by default so maybe check it when you have glitches in sharing.
http://social.technet.microsoft.com/Forums/en-US/w7itpronetworking/thread/02476acb-aa5f-4fcf-8079-4a697a065610