You've obviously got a corrupt registry. You need to get hold of a Windows 10 repair disk (if the laptop has an optical drive) or USB drive. e.g. one of these would do:
http://www.ebay.co.uk/itm/Windows-10-Home-Pro-32-64-Repair-Restore-Install-8GB-Usb-Updated-1703-/112345091260?hash=item1a284a0cbc:g:bI8AAOSwWxNY0-PV
Then you need to check that the laptop's BIOS is configured to look for a bootable disk or USB drive (as appropriate) before booting from its hard drive. So you need to get into the BIOS. To do so you need to press the relevant key - there's usually an on-screen prompt telling you which one - before Windows has loaded. Then you navigate (using the arrow keys, together with 'Enter' and 'Esc') to the boot order instructions.
Once you've configured and saved the correct boot sequence you need to power down the laptop, insert the USB drive and power up the laptop again. (If you're using a disk, rather than a USB stick, you can put it into the drive before powering down).
The laptop will then boot from the external media and you'll be offered various repair options to play with.