As stated above, it is very important you dont initiate any write actions on the disk, dont try to format it, dont try to put something else on it, you might (and probably will) overwrite data by doing this.
There are a number of tools out there which allow you to recover some or all of the "deleted" data. I suppose you're using windows?
I've had some success recovering files with a tool called Restoration, its freeware and can be found here http://www3.telus.net/mikebike/RESTORATION.html
. Its a fairly lightweight tool, but if I remember correctly it can't (or couldnt when I used it) recover partition informatiom. A big advantage of this tool is that I doesnt need to be installed, just unpack the folder and you're set.
Do keep in mind if you are able to restore data, restore it by copying the data on another hard disk, be sure not to write anything to the disk with lost data.