Found this off a google link:
"A RIS Server needs to be authorized in the Active Directory. Also, note that an RIS Server gets authorized as a DHCP server, using the DHCP administrative tool. Therefore, DHCP and RIS are logically inseparable."
I think your outta luck but it is possibly to have it running, with AD and as a Domain Controller.. but not have it affect the other machines, they'd just have to join the domain but you wouldn't have to have them get DHCP from the server, etc...
At work, the Windows Admin has a RIS server with our existing Windows Domain controller w/Active Directory but we still serve out IP's via our Linux server.. etc.
I wouldn't know any other way around it, I just know I don't blame you for not wanting it.. if ours affected our network, we'd make sure the Windows Admin doesn't have a job any more..