A year ago, I ranted about the fact that Windows will map a newly added USB drive on the first available drive letter, even if there's a subst'd or mapped network drive on that letter.
Kyle pointed me to USBDLM, the USB Drive Letter Manager. This is a piece of software that, as well as working around this bug, will allow you to ensure that bad USB devices are always mapped to the same drive letter. It's free for personal or educational use, but costs for commercial use. Not that it's really a solution - Microsoft, this is a simple bug that you could fix today. It is an exercise for the reader to get Raymond Chen to tell me why it's not as simple as I think.