With all due respect, I strongly advise against following the advice of the two posts directly preceding me.
Trustworthy software-based solutions documented and recommended by trustworthy authors have been posted already in the first two posts. Beyond that, I have already mentioned that physical destruction of the HDD is the only proper answer remaining if a software-based solution is not satisfactory.
Just sell it without the hard drive, problem solved :)
I mean, if you are truly worried about it.
What are we talking about here? Why would you need to destroy all traces of data to where only sophisticated police forensics could retrieve it? If you have only the normal worries about bank and other personal data, then there is no reason you can't run Factory Recovery to hand the new owner the PC as it came from factory, or format to do a Clean Reinstall, adding at most Disk - Clean and Clean All with Diskpart Command.
There is almost zero chance data could be recovered from either of those options. I've never been able to retrieve any readable data from a formatted HD. So everything else is tin-foil hat overkill, unless you really have something criminal to hide.
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