That depends. The usual options, depending on the make and model of the computer, are:
1. Boot from the recovery media that came with the PC and tell it to go back.
2. As #1, but you have to make that recovery media first from some shortcut they provided in the Start Menu.
3. Press the secret key (combination) at boot to boot into the recovery partition. Continue as in #1.
4. Install Windows from the Windows CD/DVD that came with the computer. Install drivers and utilities from the driver disk(s) that came with your PC.

We don't know which of the 4 applies to your computer. You're the only one who can tell.

Kees