|The drive letters do not change, once they're been set, in the operating system for internally connected optical drives in 2000 and up , unless, possibly, if you disconnect the drive and reboot at least once, then re-connect it again. |
You can assign any available drive letter (C to Z) to an internally connected optical drive in 95 and up.
In ME and below, you do that in the properties for the optical drive in Device Manager, but that assignment can be changed automatically when certain things need drive letters that have a higher priority - so the higher alphabetically the drive letter you assign, the less likely the assigned drive letter will change.
In 2000 and up, you can do that in Disk Management, and also do that with any hard drive partition except the one Windows has booted from.
Nothing can change the assigned drive letter once it's been set, unless you disconnect the drive and reboot at least once, then re-connect it again. .
If you have 4 computers, each with internal optical drives, if the drive letters of one or more of the drives are not the same, that you want to be able to insert this program disk into, you can make them the same. If drive letters of what you want to assign are not available, you can re-assign other things to other drive letters, to make the drive letter you want available.
On the other hand...
The drive letters for USB connected devices data can be stored on or read from that are assigned drive letters, including USB optical drives, are assigned according to which drive letters are available when the drive is plugged in, in alphabetical order.
Once a USB connected device has been plugged in, it will retain the drive letter assigned to it until it's unplugged.
If the drive letter assigned is not what you want, you can assign a different available drive letter in Disk Management once the USB optical drive has been plugged in, then use the program disk, but that assignment will be lost when the USB drive is unplugged.