alpianin,
STOP! Before you go any further, make a clone/copy of the hard drive. You already made a serious mistake and formatted the wrong drive. Make a copy of the drive to be recovered and work off of the copy. ddrescue is a decent tool to use to make a copy. You can get it here:
http://www.gnu.org/software/ddrescue/ddrescue.htmlA program called testdisk may do what you need to do. You can get it here:
http://www.cgsecurity.org/wiki/TestDiskBoth of these programs were originally written for Linux. There is a version of testdisk that will run under Windows. I'm not sure about ddrescue, remember Google is your friend!
How was the format done? What Operating System?
If it was Windows Vista or Windows 7 AND you managed to use the full format (default in Vista) rather that the quick format (default in Windows 7), your data is gone. The full format in both Windows Vista and Windows 7 writes binary zeros to every sector before formatting the drive. See:
http://support.microsoft.com/kb/941961. If you used Windows XP, your data is likely to be there.
If the format process took a REAL LONG TIME, hours rather than minutes then you happened to use the full format. OOPS! No hope of data recovery.
Good luck.
TonyC