Eh, what Os are you using? In win2k, use the Computer Management, click on Disk Management, right-click on the drive you want to move and choose Change Drive Letter and Path... If you have a drive currently occupying the D: you'll have to move it around a bit, but that shouldn't be too hard...
Well, then it's allmost the same... For Xp it's start -> Control Panel ->Performance and Maintenance (only if not using classic skin) Admnistrative Tools -> Computer Management and then like I described above... For a full detailed article about it, clikc here [EDIT] Spelling [/EDIT]