I'm selling a P.C...

I’d use a disk utility to do a low-level format of the drive and hand it off as-is. Let them put their own licensed OS on it and their own apps.

well he wants windows to stay on it

Get a copy of Dan’s Boot and Nuke, wipe it out, and reinstall Windows. It’s the only way to be sure.

After you wipe and reinstall windows you might also want to reinstall all the appropriate drivers.

You could reformat the drive, then reinstall Windows if you have an installer.

Yep, format and reinstall windows. Only way to be sure/safe.

To reformat drive u need software?

You can do it in the installation process for Windows. Just boot to the CD and don’t try to re-install windows when you have windows running.

Keep in mind that the reformat done by the Windows installer is not secure, and some if your data could be recoverable. DBAN does a secure clean of the whole disk.