There was a neat article i found a while back that said.
1) use a File Shredding program (there’s a free one windows makes) that shreds pictures, photos, etc into random 0110100 cominations
2) use an erasing program (also a free one windows makes) to erase all your blank data (erased data isn’t gone, so this turns all the blank or unused data on the computer into garbage 0101010110’s also)
3) factory reset the computer in the BIOS (restart and hold f8, google it)
4) if you wanna be more secure, take the harddrive and drill a couple of holes in it, or smash it with a hammer.

5) unless you did number 4, you can probably give it away or sell it to a recycling company for $20 or whatever you were going to do before.