Windows Defender, formerly known as Microsoft AntiSpyware, is a software product that helps combat malware. Windows Defender was initially an antispyware program; it is included with Windows Vista and Windows 7
But as you have already found out its not as good as it could be.
SpyBot Search & Destroy is a powerful application that targets and eliminates malicious types of programs such as malware off your computer.
It will pretty much do the same thing, i think the best way to look at it, is what results you get from it.
I use mall ware bytes personally, so i cant comment on Spybot.
But you get what you pay for.
You can have all the prevention you want or ever need but if the owner of the laptop is not up to speed then they can let any threat in at any time. Its a constant battle with tech and limited knowledge.
Normally the anti mall ware software will guide you to the correct decision, where as anti virus can give you too many confusing options if you don't understand.
Making sure you do Microsoft updates every week / month will help considerably as there is a new scan pretty much download as part of the updates every month.
The worst case is that they fall for ransom wear.
This is when your asked to pay money to unlock your pc.
May be an idea to turn on parental controls if they do not understand and create an Admin account for your self.