There is literally no limit to what can be done. The command prompt in the video is just an easy demonstration, and even with that alone you can pretty much do anything you want.

If you can hijack someone's computer like in that video, uploading a trojan or virus is trivial. If your computer gets compromised in this way, the person on the other end can do whatever they want to. There really is no limit, and if they are smart about it, you won't even notice that they hacked you.

The best protection is what nimrod mentioned: don't run your computer in admin mode.