Restore Windows 8 / 8.1 Registry when computer will not boot

[sjj1805]

If your computer refuses to start up and the reason would appear to be due to a fault in the registry (Often due to an updated driver that windows doesn't seem to like! or you tried one of those "Registry cleaners" and it totally messed up your computer)
You can restore the windows registry to a previous state.
Firstly you need to access the hard drive by whatever means possible.
This could be by booting up from a USB stick with a program such as Hirons Boot CD/USB or placing the hard drive into another computer as a slave drive, or if you have a multi boot computer boot up from one of your other operating systems.

Having gained access to the hard drive you then navigate to
(Broken drive letter):\Windows\System32\config\
Here you will find your current registry files which are:
DEFAULT
SAM
SECURITY
SOFTWARE
SYSTEM

Now rename those files - for example:
DEFAULTx
SAMx
SECURITYx
SOFTWAREx
SYSTEMx

Now look at the directory and you will see another folder "RegBack"
(Broken drive letter):\Windows\System32\config\RegBack
Here you will find previous copies of those above 5 files. now simply copy/paste them
to
(Broken drive letter):\Windows\System32\config\

Now try and boot up Windows again.