Hi Sax,
You either take the drive out and then put it as a 2nd drive in another machine and use disk wipe software from there, or you use a piece of software that can create a bootable cd for your machine and then wipe the drive from that. I'm guessing you probably want the latter, free software would be something like DBAN (
http://www.dban.org/ NB well worth a read of the documentation/faq first) - though there are probably others out there too (when they get really serious they start to become costly though). Keep in mind you obviously will nuke the OS as well, something probably a charity shop will have no idea what to do with so hopefully you have a reinstall disk for the OS

Other products I know of are killdisk (though I think it only gives a 1 pass wipe at the demo version, you prolly want more than that just to be safe) or I think ccleaner appears to have a drive wiper as well (but looks to me like you'd have to do the first method i mentioned and put the drive into another computer).
In short, if your wiping your HDD you're prolly doing allot more than the average punter does (unfortunately), which is great

I'm no expert on data recovery but I think if people have some really specialist tools and allot of time I think they could even get some of your data back, but unlikely they'd go to that much trouble. There is software that does complex passes and many many wipes but usually you find they cost

.
Hope that helps anyways, I'm sure Cern or eko can maybe add something too or maybe some better products !
las