How to upgrade windows 2000 domain controller to windows 2008?

I am using a single domain controller on Windows 2000. It is a basic DC and it works fine. Now there is a second server that also has the same server version. I am running Exchange on it. The version is 5.5. Now I am planing to go for upgrades. I am planing to go for upgrade on the DC and install all latest version of Exchange server. On the old server there are around 100 Exchange user that I am planing to migrate them. What should I do. Should I install the latest version of Exchange on existing server or simply upgrade everything by fresh installation.

You must not directly upgrade Server 2008. Because there are existing members on the same. You have to follow a proper way for upgrade or there might be errors. Now you cannot directly migrate. It is possible if both the system are different and if you upgrade there are chances that everything will be updated on its own.

Try to make those changes on a new system. Because it will be possible for you to migrate the settings. That is an easier task. It is not complicated at all. You can use a migration wizard later on to move all the settings, users, permission, etc to the new server. On existing it will be complicated and also no easier.