CCM has it's own OS W2K image that's required by use per Cisco, that's already been configured and such to run with the CCM application. Unity can use either the server manufacturer's W2K image or a straight install of W2K from CD. There are other differences, but that's the one that comes straight to mind (aside from the fact that the two programs are so fundamentally different, and just to run CCM 4.X by itself you'll need bare-minimum 1 GB of RAM).
It's never a wise idea to run Unity and CallManager on the same server; you'll just run into unnecessary headaches. I have a test lab made up of four Dell PC's - one for Unity, one for the Exchange back-end, and two CCM's, one publisher, one subscriber. All have 1 GB of RAM and at least a P4 1.5 ghz processor. A bit slow, but they all work exactly as they should...
TMH