I am planning to use the hard drive in my current computer in a new system im building, so im also planning to format the drive for the system and i have some questions about it. When you format a drive that already has an OS on it (windows 7) will it delete the OS making me buy a new product key? Or will the OS stay on there.

OS will be deleted with a format. You can install windows using a USB or disk back onto the drive after installing it into the new system, and use the old product key as long as the previous computer that had the OS in it no longer needs it (its dead etc). Might need to call microsoft to reactivate the copy though.

Ok, thats good news. Thanks for the reply