What to do if the CD doesn't boot
All CDs given out at Software Freedom Day are bootable. However, being able to boot from CD requires your computer to look for a bootable CD at startup time, BEFORE looking for a hard disk. Let's face it, unless something is seriously wrong, it will find the hard disk!
If the computer ignores the CD, and boots from the hard disk, reboot the PC and do some work. Normally you would pay no attention to the messages that appear before Windows starts. Now it's time to look at this text...
Look for something that says something like Hit <F-Something> for Setup or for BIOS Settings. You have to be on the ball here, and do this fast. If Windows starts booting up, you're late. Try it again. Note that the key to get into these settings vary from computer to computer, but will usually be one of Del, F1, F2 or Ctrl-Alt-Enter (on some Dells).
If you can't find this key, look up BIOS or setup in your computer manual or ask your supplier.
OK, so you've got into Setup
Look for something called Boot sequence or similar. Make sure that CD comes before HD. Often the markings will be something like FDD->CD->LAN->HDD meaning look for a bootable floppy first, and if you don't find one, boot from CD, and if there's no bootable CD, see if there's a network boot available (there will not be at home), and finally look for a hard drive to boot from.
Saving the changes
Again, the key for doing this varies from PC to PC, but normally it will F10 or something on the menus called 'Exit and save changes'