Working with SMB in Windows 2003 domain

I have a Windows 2003 domain with a mix of Windows XP and 2000
computers. Since our company is spread across 7 different offices over
slow WAN connections, I'd like to disable SMB in order to speed up file
access in the offices which do not have servers. I know this change
probably won't make a huge improvement, but anything would help. What
I'd like to do is require our servers to continue using SMB, but
disable it for the client computers. I know SMB changes can be
devastating if done improperly, so I could use advice. Any suggestions
on how to do this?

Thanks in advance, Brett