Another vote for DBAN. Download the ISO, burn it to CD, and boot the computer from it. If you run the standard DoD wipe, the data will be GONE. Even the CIA/FBI/NSA can’t recover it (this is the same wipe the military uses when disposing of old hardware). It’s one of the few options that civilians have for full drive wipes that actually works, and may be the only one that’s free.
Be aware that DBAN is sloooow though. I’ve used it many times when disposing of old hardware, and it seems to run about 5-10 minutes per gigabyte. Wiping a modern, high capacity drive can take DAYS. I usually just stick the computer in a corner, fire it up with DBAN, and check it every few hours to see if it’s done.
DBAN is an excellent wipe product.