How to recover os on formatted hard drive

Pay an extremely large amount of money to a data recovery company and accept you will only be able to recover some of the data

restore from a backup copy you should have

Buy another OS.

Was the hard drive partitioned to have a recovery sector? If so, you may be able to restore the factory settings.

Buy another OS? If this machine is a prebuilt tower, if you look on it, usually on the top or on the side, there should be an OEM key that was applied by the manufacturer of your pc. For example, if it's for Windows 7 Home Premium, if you have any friends that have a standard vanilla Windows 7 Home Premium disc, you can use that disc to reinstall Windows 7. Just when you reinstall, you install using the cd key affixed to your pc. You will have to go through trouble of finding drivers and what not, but the key attached to your tower should activate and be legal for you to use. If you take the pc to a repair shop, expect to pay a diagnostic of some type and then 100 bucks or a little over for them to install the OS and drivers for you.

If you can't get a hold of the disk make a usb installer following these instructions:
http://www.pcworld.com/article/248995/how_to_install_wi...

You'll need to note what version of windows your product key is for - then install windows as normal.

You may need to change the boot order in bios so that you can boot from a usb.

Sorry I assumed that the OP would know that there was a sticker on his PC if it was premade. Im guessing it wasnt and he doesnt own a copy of the OS. Thats why I said buy it.

The OP must have a disk , or the boot drive could not have been formatted .

There are ways to format a drive without a disk. He said he formatted the hard drive and installed a different os and wants the original back. So to avoid paying for another os there should be a recovery sector if he didn't format the entire hard drive or he can download the os and make a usb boot drive and use the product key on the sticker. If he doesn't have the sticker or a recovery sector though He'll need to either buy another os or live with the one he installed.