Computer doesn't ign a letter to new external hard drive.?

Just recently bought a new Ultra Speed external 640GB drive and when I plug it in and turn it on, the computer recognizes it but doesn't show it in the 'my computer' folder. I tried igning/changing the drive letter in sys mgt but it doesn't it shows unknown address or something. Is it possible that the disk letter could be hidden somewhere in windows or something? I looked at the cd that came with it and it tells me that I don't need to install any drivers for it since I'm using XP.