Does anyone know what to do when the computer will not
boot and the monitor only shows the message "Operating
System not found"?

you need to use the repair option that is on theWindows XP
CD.To do that you must make sure that your CD-Rom is set in
the bios as the first bootable device,put the XP cd in and
reboot your computer let begin to load when you get to the
option that has 3 options,hit enter when you get to the
license agreement press F8,then you will come to options
that will ask you "what selected windows files would you
like to repair" press "R" this will repair damaged files,
let it run the complete installation you will not loose any
previous files from your original installation.