With the drive connected to your computer follow these instructions.
Windows XP:
Right Click "my computer" (XP) or "Computer (Vista/7)
click manage
select "Disk Management"
wait a minute for it to load.
You should see 2 rows of information
top will be a list of drives
bottom will be a visual representation of these drives.
on the top row where you see the drive letter and name, look for your external drive.
check to see if it has a drive letter, if not right click it
select "Change drive letter and paths"
click "Add"
Make sure "assign the following drive letter" is selected
choose the drive letter you want it to be
hit OK, OK again, and check to see if you can see your drive.
If so, close the disk management window, you're done. If not, report back here.