Original Source:
http://www.2brightsparks.com/help/modules/KB/html.php?category=3

As described on the above site, it didn't work for me, so I've modified the following to reflect what did work for me.

Assigning a permanent drive letter to an external USB drive / flash drive / memory stick in Windows XP

Introduction

If you'd like Windows to use the same drive letter every time follow these steps:

- Start (point at "my computer" and right click
- Select "Manage"
- Expand the item Computer Management (Local) on the left (under Console Root)
- Expand (if needed) the item Storage then click on the Disk Management item
- When the window fills on the right side of the console, locate your USB drive in the listing on the lower right scrollable box.
- Right-click on the drives box and select Change Drive Letter and Paths...from the pop-up menu
- Here is where you either assign it an unused drive letter or edit the currently assigned drive letter.
- From the Window that appears, you can choose the drive letter to use. Click OK once you've added/changed the drive letter.
- Select File -> Exit from the main menu, and save the settings when prompted (or not, it doesn't matter) or simply close the window.

You can test the changes by removing the USB device and checking in Explorer to be sure it is no longer available, then plug it back in and the newly assigned drive letter should now be present (F5 to refresh display).

When the USB drive is plugged in, I see that _sometimes_ it just assigns the next drive letter - but _usually_ it gets the drive letter assigned by this method. If it didn't get the desired drive letter, I simply unplug it and plug it back in.