These are the steps I usually do to fix my Windows problems:
0/ Scan for malwares with
zemana portable version 2 (1 full scan and 1 custom "Users" scan) and
NPE
1/ Find the error code (by bluescreenview and Event viewer) and follow the instructions NOT from Microsoft engineers. The instructions from users are much more specific and reliable
MS engineers simply copy and paste their answers and usually they never fix anything
2/ Run "
sfc /scannow" and
check disk/repair for drive C:
3/ Run "
DISM /Online /Cleanup-Image /RestoreHealth" and then
sfc /scannow", with or without
sfcfix
for windows 7/vista, dism alternative is "
System Update Readiness Tool" or CheckSUR. It's the same as dism
4/ use "
Tweaking .com - Windows Repair Free portable" and perform some repairs" -> it may create other problems and may fix your problems
5/ Use "
Wise registry cleaner" to clean up registry and then, registry defragmentation -> registry defrag DID fix some unfixable problems for me!!! and magically sped up my old PCs for unknown reasons
6/ Use "
Last known good configurations" during boot -> it did fix a very serious BSOD for me yesterday (unbootable, only safe mode was bootable) when everything I tried above failed. Registry error 51 bsod
This is a gem that most people forget about. It has saved me countless of times
7/ Use
System Restore or
image backup
8/
Clean install Windows with a USB flash drive if there is no backup. Wipe the whole partition, I don't install on top of the previous version
9/ if everything fails, hardware intervention might be needed