If so, you're best off going to the manufacturer to see if you can get the disks to install the OS and whatever came with the factory version. They'll need your model # and will also want payment for the disks. It won't be as much as to buy a retail version of Windows but they don't tend to give these out free. If additional software was purchased by the original owner and that was on your machine, you'll need to buy that yourself separately.
There are some programs that came with my Windows 7 Pro. that will not work or be reinstalled? I don`t have the CD for my Windows installation [used pc]. Do I need to buy a new CD to reinstall all the Windows programs that come my Desktop?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic