most older computers (500mhz or less) did not have a boot from cd option in the bios and thus could only be booted from floppy drive to install xp.

if you had a boot virus or for some reason windows could not boot than you can use the xp boot disc to access the recovery console and perform tasks in a command menu to solve verious problems.
check it out here

if you would like to learn how to create a real windows environment boot disc that loads the windows shell as well as drivers and internet explorer and back up programs and disc utillities etc.... try reatogo
http://www.reatogo.de/