You can the drive letters of your harddisks and cdrom/dvdrom drives by opening the Server Manager, expanding Storage and clicking Disk Management. Now you can right click on the drives and select Change Drive Letter and Paths from the context menu. If a drive letter you want to assign to a drive has already been taken, remove the letter from the drive it is currently assigned to, assign it to the drive you want and assign another letter to the drive that currently doesn’t have any driveletter.
It is not allowed to change the drive letter from the System disk. I had this problem and just physically disconnected all other harddisks, installed Windows Server 2008 and re-attached them. I couldn’t find another way to do it.
I couldn’t find this topic in Windows Help and Support, but I hope this explanation is enough. 😉