Thats very simple man. Which OS r u using?
On XP, just right click on My computer, click on manage. In the new window that appears click on disk management on the left. Disk management should open. U can view all the drives u have on the computer along with the partition info. Now just right click on the partition or CD drive that u wish to change the drive letter for, in the menu that appears, u will have the option of changing drive letters.
The same procedure for Windows 2000.