I know this is not the answer you're looking for, but there is no way to be 100% sure you're erasing all the data unless you wipe the drive and install the OS from scratch. There are 3rd party tools which claim to do this, but there are so many places that data can reside, and so many various registry keys that contain information, that it is very hard to guarantee everything has been deleted. And if you do delete the data to what seems like a satisfactory level, make sure to defragment the drive as completely as possible, because undelete/uneras
It may be worth investing in a good external optical drive. If you deal with a lot of laptops, it's something you'll probably find a use for many times. Also, the laptop's manufacturer will likely sell you restore media for $25 or so, or you might have luck purchasing it on eBay, etc.
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.
Click on the Windows button, in the search box type “Administrative Tools” & double click on Disk Management, This display you’re Disk Partitions.
If you want to remove all your personal details from the system you should reinstall your operating system & format your hard drive. If you are selling your laptop it is best to format your hard drive a few times to make sure your details can’t be recovered by specialist software.
2nd year CompTIA A+ student &
City & Guilds IT Practitioner