someone would have to go looking for it and even know where to find it amongst all the bs they find from recovering your hard drive. If you wiped the hard drive and reinstall the OS which is replacing at least that space with more info there are seriously .000000000000000000000000000000001% of people that could find something on your hdd of any value to them. Additionally they would have to target you through that hard drive. If you are donating the machine to somewhere they are going to constantly be adding and erasing layers of stuff to that hard drive until it eventually fails or gets tossed out.

You probably have a better change of winning the lottery and while exiting the 711 with your winning ticket getting struck by lightning while a 45 foot giant mechanical shark chews your arm off..

Im not saying to be stupid with your data but someone is going to have to care AND have the ability after you have done your due diligence.

That being said I have a hard drive shredder at my work . But I would never throw a whole computer away, take it to an electronics recycler so it doesnt end up in a landfill if thats your concern.