armageddon

Depends on the file system you use, FAT32 for example, marks where to find the information (data) on the harddrive called the File Allocation Table (FAT).  Its much easier to recover the data if that guys just simply erase your HD without puting back anything. Your chance of getting back your data is... well.. slim... if he already installed another OS on top of it.  Getting your data back is much easier than getting back your FAT, once your table is lost it makes no difference even if you get back 100% of your data, because you won't be able to tell the difference between the files you recovered.

If you used NTFS file system, its a totally different story.