Booting of a CD

This should be a fairly simple question.

If you are attempting to fix a computer with a windows OS (2k,XP,Vista) and the software as degraded so that the OS will not boot who do you go about getting a boot disk. I am repairing computers and do not own a copy of installation CD’s for the ubove mentioned OS’s. Do I have to purchases these disks or is there another way or place that I can get copies of the boot files. I have looked on the internet but I am having troubles.

Cheers,

Corey