For the most part removable drives will take the next available drive letter. However, to set the drive letter (at least before a reboot, I haven't tried after) you can go to Control Panel>Administrative Tools>Computer Management. Click on Storage>Disk Management and you should see your flash drive. Right-click on it and select "Change Drive Letter and Paths." I would advise using something far down in the list to assure you won't conflict with other drives that may be plugged in first.
Edited for clarity