HD Enclosure Not Being Recognized

My laptop stopped working so I went to get new one and took out the hard drive to put in an external enclosure. Old laptop ran on XP Home. New laptop runs on Vista. When I connect by USB, the new laptop running Vista recognizes the hard drive (goes through the installing new hardware and drivers messages) but does not assign a drive “letter” so I can’t figure out how to access it since it doesn’t show up as a device under “Computer” (but shows up in the Device Manager). Please help!