You know how annoying it can be when your computer is acting up. Maybe programs keep crashing or the system is running much slower than it should. A corrupt driver could be causing it to crash before it even starts.
Maybe the problem is faulty RAM. It could be you forgot your password and you're locked out of your account. Hunting down the problem isn't a cakewalk, even with the right tools and instructions. But that's still better than the situations where you can't use your computer at all.
Whatever the reason, that's the time you need a diagnostic boot CD handy. This loads up a separate operating system with pre-installed tools you can use to troubleshoot your main system. Fortunately, I know of one you can add to your "just-in-case" drawer.