I would normally suggest 1 of 2 things. Ether getting a new HDD and installing a fresh install of the OS (old drive gets physically shredded). The other is using a utility such as D'Bans Boot and Nuke, to rewrite every bit on the drive to Zero. Both are Military grade standards for destroying sensitive computer data
But that doesn't solve your problem, you can't reinstall, and you want the Original OS intact.
You can try Microsoft's SDelete Utility to delete sensitive files
For mac users you can try Permanent Eraser
It all depends on how sensitive your personal data is. If there is no critical data on your computer, like banking information, tax info, or information that would be considered confidential for your work The above utilities should work fine for you.