First of all:
DO NOT, under ANY circumstances, worry yourself about getting Windows to show a drive letter on that drive. You will only make things worse, because at that point you'll be writing data to it, overwriting files that are already there.
Use the disk recovery tools listed already. You could also try a few Linux Live distro's, or maybe even the Ultimate Boot CD out. There's a very good chance you can recover your data, as long as you DO NOT write to the drive at all.
I lost 1.5TB worth of information. Copied it all over from my dying 1.5TB drive to my 2TB drive. Then pulled the 2TB drive from its enclosure, tossed the enclosure, and put the drive as an internal in my machine. The stupid enclosure encrypted the data, and I reenabled the drive in Device Manager.
Recovered it all
. I was stupid and paid recovery costs at a local PC shop. $400 was a small price to pay though for my 1.5TB worth of information.