Way back in the 20th century, Windows prepared you for the day your PC wouldn't boot. It came with a program that formatted a bootable floppy disk, complete with diagnostic and repair utilities. If you had the forethought to create that floppy while Windows was still working, you were ready when it eventually failed.
Alas, the Windows Boot Floppy went the way of DOS (the operating system it actually booted). Modern versions can't make that floppy (they can format a bootable DOS floppy--if you have the drive--but without utilities), and DOS can't handle NTFS hard-drive partitions.
Since Microsoft doesn't supply you with the ability to create an emergency boot disk, others have stepped in to fill the vacuum. Here are six worthwhile emergency boot CDs, all downloadable, and most of them free.
Yes, I know you can't download a CD. Most of these packages come as .iso files--easily burnable disc images. If you double-click an .iso file, there's a good chance that a program you already own--perhaps Nero Burning or Easy Media Creator--will come up and burn it to CD. If that doesn't happen, download and install ISO Recorder.
Know Your Rescue OSs
Since DOS doesn't handle XP or Vista repairs well, each of these discs boots into one of the following three operating systems. It's good to know a little about them.
Windows PE: The official, CD-bootable version of Windows (the PE stands for Preinstallation Environment) makes the obvious choice for this sort of thing. Unfortunately, Microsoft maintains strict control, and few utility authors have received permission to use it.
BartPE: Since Microsoft won't share its preinstallation environment, Bart Lagerweij created his own, and he gives it away for free. But to avoid copyright infringement, he can't give you everything you need to create a BartPE disc. The missing elements consist of Windows XP installation files you may or may not already have.
Linux Live CD: The name refers to any version of Linux you can download as an .iso file and boot off a CD. But Linux can be an intimidating environment for the uninitiated, it doesn't always handle NTFS well (many versions can read NTFS but not write to it), and it is seldom geared to help with Windows problems.