You should be able to do this easily -
In Vista, you can use the Disk Management console to manage the drives on your computer. You can assign any letter between E and Z to a hard disk drive while letters A and B are reserved for floppy disk drives C your boot drive and D usually for a CD/DVD drive.
To access the Disk Management console, right click Computer and select Manage. Under Storage, click Disk Management. The drive configuration of your computer will be displayed in the details pane. You can change the drive letter by right clicking any volume and selecting Change Drive Letter and Paths. Click the Change button and use the drop down arrow to select the drive letter you want to assign to the volume. Click OK. Click Yes to confirm your actions.
Make sure that your external drive is plugged in before you do this so that it will show up.