1) Click [Start] -> [Settings] -> [Control Panel].
2) Double-click on the [System] icon, then click on the [Device Manager] tab.
3) Locate your [CD-ROM] drive in the list, bring up its [Properties], and click on the [Settings] tab.
4) In the section at the bottom of the dialog box, enter the desired drive letter in the boxes labeled [Start Drive Letter] and [End Drive Letter] (for a single CD-ROM drive, these letters will be identical)
After the drive letter had set, the computer will prompt you to restart to let the changes take effect.