DONOT TOUCH THE HARD DRIVE.
DONOT BOOT TO THE HARD DRIVE.
DONOT USE IT AT ALL.
your data is (for the most part) safe right now.
tell us more about the hard disk.
what OS you were using. what was the original partition scheme you were using (eg, C: D: E: or C: D: G: H:, etc)
please dont use that disk for anything, as any sort of usage (including trying to install software/os) can result in destruction of data.
i'd advise you to get an external or 2nd hard disk, and arrange for a live CD of a linux distro (like ubuntu). this will ensure that the disk is not used till the data has been safely copied.