Crisis Aversion Tool (CAT) is a portable, open source tool which makes it easy to fix a host of common Windows issues.
Is Windows Update no longer working, for instance? Click the Fixes button, check "Reset Windows Update" and click Apply, and the CAT will do its best to repair the problem.
Maybe Task Manager won't launch? This can happen if it's been disabled by malware. Give your system a thorough antivirus scan, if you haven't already, then use the "Reenable Task Manager" option to regain access to this useful tool.
And there are other options to reset your networking interfaces, restore many Explorer defaults, fix Internet Explorer, solve most printer installation errors, and more.
Programs like this do need to be treated with extreme caution (especially when they're still in beta). Recovering your default settings has its problems: you may get Windows working again, but there's also a chance that you could break some installed programs entirely, perhaps making your situation even worse than it was before.
As long as you only apply fixes when you're sure they're necessary, though, one at a time, and after making a complete system backup ("just in case"), then the CAT could save you a lot of time and hassle. And it's certainly deserving of a place in your portable troubleshooting toolkit.