Most of your data is probably recoverable. Here's an overview of what needs to be done:
First of all, as another poster said, copy all sectors of the drive into an image file as a backup. Windows cannot do this natively; you'll need some third-party tool to do it in Windows. If you know Linux at all, this is dead simple to do with the 'dd' command (but its joke name isn't "destroy disk" for nothing -- dd is dangerous; be careful!).
Then you need to:
- Find out whether you nuked the partition table, or just reformatted the same partition.
- If you nuked the partition table, you need to reconstruct it. The easiest way to do this is to actually use some partitioning tool and just make the partitions the way they were before. If you don't have a record of the exact layout, you'll need to find the partition offsets by looking for the first block of each filesystem. Google your filesystem to find out what its signature is. Then you need to do a bunch of math.
- If you didn't touch the partition table, your job just got easier and harder at the same time. The good news: no partition table math. The bad news: you nuked the master file table, so all your files are orphaned and not at all easy to recover.
- There should be a backup copy of the master file table somewhere. Since you're using Windows, I'll assume it's FAT or NTFS. Either of those only have one backup. If it's FAT, you're probably screwed, because the secondary FAT sits right next to the first FAT and would have been clobbered right away by the format. If it's NTFS you might have slightly more luck, but there's still a good chance that the format got both copies of the MFT. If you can find it and restore it (by copying it back onto the first MFT location), you're in luck -- your files should all be waiting for you next time you mount the disk. If the MFT was fragmented, this is probably not going to be easily doable, so you'll have to find a tool that can use the backup MFT to browse the filesystem -- I recommend one called DiskExplorer, which I've used to restore files from exactly this situation.
- If the backup file table is wiped, you can still get at your data, but now you're down to finding it one file at at time based on file signatures/content. Any file that was fragmented is probably unrecoverable (you'll get the first piece, but good luck finding any subsequent pieces). There are tools available to look for specific file types. Recovering smallish files with easily findable header signatures that are always the same is easy -- you will get all of your JPEG's and Word documents back. Good chance any music files are also recoverable. Recovering larger files (like movies) is, as I said, unlikely if they're in more than one piece, but since this is an external hard drive that presumably didn't have an OS running on it before, there's a fairly good chance most of the large files are in one piece unless you've really done a lot of copying and deleting on that drive. Recovering things like text files, which don't always look the same at the beginning, is harder, because you'll have to find each one based on its contents.
Good luck,
~Felix.