Drive letters are assigned arbitrarily by Windows and the same applies to you plug-in your devices.
It is however possible to force the assignment of a letter. To do this simply go into Control Panel/Administrative Tools/Computer Management.
In the left pane select "Disk Management" and then in the right pane click the left button on the drive you want to change the letter and choose "Change Drive Letter and Paths".
Note that: You can not change the letter of a partition system (e.g Windowscan't be installed on D: instead of C:)
If you can not change the drive letter of a partition (not system), try to perform the above manipulation under Safe Mode.