Apparently you can force Windows Defender back on by editing a registry key, AFAIK Windows Defender was superseded quite some time ago by Microsoft Security Essentials, and therefore will be turned off (or not even installed) if MSE is on there. (That's my understanding of it anyway, none of our PCs have Defender installed and I honestly can't remember the last time I was aware of its presence.)
As such I'm not sure you should even try to turn Defender back on.
Quote:
x32:
HKEY_LOCAL_MACHINE\Software\Policies\Microsoft\Windows Defender
x64:
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Windows Defender
Delete or change the key value of "DisableAntiSpyware" to 0
I'd be inclined to reboot in safe mode (keep tapping F8 from the BIOS screen onwards) and once in Windows run any and all AV/anti-malware software you've got against it, rather than from within 'normal' Windows. Spybot Search & Destroy and Malwarebytes which you seem to already have should support this, and MSE with the latest definitions.
If everything comes up clean after full scans in safe mode there's probably something else going on.
Looking to the future, a great way to keep Windows PCs clean with non-computer literate types is to give them normal user accounts and not Administrator accounts, which massively limits their ability to fuck things up. Just get everything installed that they need with an Administrator account, and then give them a normal user account for everyday use.
EDIT - I assume Windows Update is turned on and the laptop is fully up to date with all Critical and Important Windows updates?