Determine the type of problem
Windows Not working (unknown reason)
Smart check (20% it is a bad hard drive)
Check disk
SFC /Scannow
Hijacked - popups locked up, browser hijacked etc.
Create restore point if possible
Disable all start-up items (regedit disable startup items)
Purge all temp files - restart to delete locked files
Restart in safe mode with networking (enable hidden administrator)
Start services required for installation (command line)
Download malwarebytes, hijack this etc.
Now run your scans and fixes.
It is a great collection, but so many people attempt to repair Trojan/Virus when it is a hard drive.
A greater percent are in the temp files running in startup.