Sold my PC on eBay, what do I need to do to ensure all of my data is wiped off the PC?

[Upgradeinfo]

I recently sold my gaming PC on eBay and I need to send it off by tomorrow. I have data installed on both the HDD and SSD which I want wiped off before I send it.

How would I go about doing this? and how will I ensure the operating system (windows 7 professional) stays on the PC for the next user to start using it?

Thanks for your help

edit: Windows is installed onto the SSD not the HDD, if that helps

 

[mudpuppet]

Since a format doesn't exaclty wipe all data from the drive, your only real bet would be a HD boot/nuke. After that you would have to reinstall the OS. Otherwise, the only real option is format it, reinstall, and call it good enough. Either way, you have to have a boot disc, so your first task would be to find your old install disc/drive, or make a new one. If you have a legit copy of Windows, you should be able to download the iso file here and then burn it to a disc/drive: https://www.microsoft.com/en-us/software-download/home.