If you ever find yourself needing to diagnose a sick PC, one of the most useful tools is the Ultimate Boot CD. As the name implies, it's a complete bootable operating system (based on Linux, naturally) with a whole host of pre-installed troubleshooting tools for when your computer won't start up properly into its installed OS.
Just download Ultimate Boot CD from http://www.ultimatebootcd.com/ and burn it to a CD or DVD disk. Then boot the unhappy computer from that CD to gain instant and direct access to all the useful tools. You can check CPU, memory and disk, and access the file system if you need to delete a dodgy file or driver that has been preventing Windows from starting normally.
And if the computer is so messed-up that a reinstall of Windows is the only option, Ultimate Boot CD lets you get at the documents on the computer so that you can copy them elsewhere before the reinstall.
Ultimate Boot CD is a 624 MB download. Is it malware-free? Almost certainly, but because it only ever boots directly into a separate Linux installation it doesn't actually matter. It's free. And it was last updated in September 2016, so if your existing version is older than a couple of months you should probably refresh it now, while you still have a working computer from which to do so. You never know when you might need it.
For people having problems with download, here is a bit of information, which should be helpful.
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