When you're regarded as the resident PC troubleshooter for friends, family and colleagues, it's important to have a good selection of tools to hand for when you need to sort out someone's computer and retain your reputation. And assuming that the PC is sufficiently OK to be bootable and to have internet access, this is generally pretty easy. Just download what you need. Or, if things are looking bad, you can at least copy any important files off the computer via the network.
But what if the Windows installation is so corrupted that the machine won't even boot? You can reinstall Windows, of course, but by the time you've also reinstalled all the applications too, it's a task that will take a long time.
One of the best ways to diagnose and examine an unbootable PC is to use a bootable rescue disk (normally a CD, DVD or a USB stick). Booting from the rescue disk gives you a usable, albeit minimal, operating system from which you can check out the problem.
One really useful such tool is the Ultimate Boot CD from FalconFour. It comes in the form of a downloadable file in 7Zip format. You'll need to extract it, and then burn it to either a CD or DVD. Then just reboot the PC from that disk, and you'll have a basic operating system pre-installed with dozens of tools to help check out the machine. Assuming the hard disk is readable, and it's only Windows that's broken, you can copy data from the broken machine over the network (there are drivers for wired and wireless networks included), and also access the web via a portable copy of Firefox that's also present. There are also loads of other useful programs installed to help troubleshoot the computer, none of which write to the hard disk so it's perfectly safe.
FalconFour's Ultimate Boot CD is a rather large 670 MB download from http://falconfour.wordpress.com/tag/f4ubcd/ (follow the link to the v4.61 download). It's a safe site, according to Web of Trust, and my own antivirus software suggests it's safe. VirusTotal was unable to scan it because of its large size.
Definitely one to download, burn to CD or DVD, and keep safe for when it might be needed in the future. And of course it's totally free of charge.
Note: This disc won't work on Windows 8 PCs, because of the different hard disk format used.