I'm returning my laptop and I'm just going to go through the Windows installation via USB, delete the partitions through there, and reinstall windows. It's an HDD, in case that matters.
That's enough for ensuring that no one can somehow regain access to the data that was deleted, right?
This isn't enough, at least for someone determined to get your data. Their are many different tools that'll do the job, I've heard good things about Disk Wipe. Its portable, free, and simple. You can get it here
No, it's almost nothing and most of data can be restored.
The best if you use specialized tool, like shred. It can be found on may linux distributions. There is simple howto:
https://www.howtoforge.com/how-to-securely-destroy-wipe-data-on-hard-drives-with-shred
