If you have an OEM computer (pre-built) it probably has a factory copy of the OS installed and you initiate a restore of that with one of the Function buttons during bootup.
You still need to copy either the data (Documents folder) or the entire drive. For the entire drive, if you have a Seagate or Western Digital (even if its USB) use the free version of Acronis True Image (for WD get it from the WD site for hard drives->software). You'd have to install on a different computer and create a BOOT DISC. Use a DVD or CD and use imgburn from www.imgburn.com .
Then restore the original copy of Windows.
*Step-by-step (if possible):
1) boot to the Acronis disc (change boot order to DVD before hard drive if needed)
2) create a backup image to a USB hard drive or second internal hard drive. Use the VERIFY option as well. Use the first or second compression level.
3) boot and restore the installed image of Windows (F-Key) if possible.
4) start updating Windows, add programs, updated drivers etc.
5) copy back over important data (usually in "My Documents" folder)
There are several ways to accomplish your task.
*I strongly recommend you start ANY of these tasks by creating an Acronis True Image backup to a second hard drive. No matter what you do, you can always restore it or at least browse it via Windows Explorer to copy back data.