Let me get this straight:
You installed DNS Server and now you are not able to see the Users and Groups section under Computer management?
Did you change the servers network configuration to point to itself versus some other DNS server?
Try managing the computer and type in the IP versus the NetBIOS/fqdn. If that works, you didn’t set your IP in the DNS, your network connection is not pointing to the right server, or something is wrong with your DNS setup.
There should not be any problems running the two services on the same machine, though I wouldn’t recommend it. A DNS server, or any Internet facing computer, shouldn’t run extraneous services, especially one that can give you direct control of the computer, like Terminal Services can. You’re also running IIS and a Web server on the same computer, so you’re just adding on a lot of vulnerability potential. If this is serving out DNS entries for a single corporation to the Internet, you shouldn’t have to change the DNS often enough to warrant the risk involved. For a situation like that, you’re better going to the console to change your entries.