Archived from groups: microsoft.public.windowsxp.hardware (More info?
START>CONTROL PANEL>ADMININSTRATIVE TOOLS>COMPUTER MANAGEMENT
Click on Disk Management
Right click on the drive you wish to change [any but C Drive] and you can assign it a Letter which is not yet used.
For a USB drive to be 'U' for example, you would scroll down to letter U.
This drive will always be the U drive when reinserted.