Linux-based system recovery distros are arguably the best bet when it comes to sorting out a computer issue where the operating system isn't playing nice, regardless of whether it's from Microsoft, Apple or the world of open-source. That said, Windows is the usual suspect, though not always because it won't boot; sometimes, you just need to run a few troubleshooting programs to diagnose and repair. Which ones? The GEGeek Tech Toolkit takes care of the selection process by providing you with basically everything.
The 1.6GB, LZMA-compressed file can be downloaded from the GEGeek website, sadly with that many bits coming down the pipe from what appears to be a provider unable to handle the load, don't be surprised if the download fails a few times.
Once you do get your hands on it, some 300 programs will be available to you -- all portable versions so you don't have to worry configuration issues or leaving anything behind when you do your troubleshooting. As gHack's Martin Brinkmann notes, once uncompressed, you'll find each program in its own folder and includes stalwarts such as CCleaner, 7-Zip, Revo Uninstaller, Malwarebytes and even Firefox.
Keeping the tools up-to-date is taken care of too, thanks to a little program called Ketarin -- quite the godsend as redownloading the archive is not a happy thought.
Personally, I'm a fan of curating my own selection of programs and I'd want something that could boot by itself to handle worse-case scenarios, but for those who like to take the shotgun approach to their diagnostic exploits, the GEGeek Tech Toolkit has you covered... and then some.