It is as simple as this (As long as it is Windows XP):
1.) Click on the Windows Start Button.
2.) Click on Control Panel
3.) Click on Performance and maintinance
4.) Click on Administrative Tools
5.) Double Click on Click on Computer Managment
6.) Click on Disk Managment
7.) At the Bottom of that screen, right click on the drive that you want to assign a letter to, and select Change Drive Letter and Paths..
8.) Click on the Drive that you are assigning in the pop up dialog.
9.) Click Change
10.) In The drop down box that is shown, select the letter that you would like assigned, then Finaly,
11.) Click on okay.
You have just reassigned the drive letter for that drive..
I am a pro DJ, and have my computer that I use in show set up with ten USB external HD's that are all assigned this way, so that when I start the machine up, they are still the same drive letters....
-William