MS made a deal with the Manufacturers to provide XP as an alternate to Vista. It
is charged to the manufacturer, but is supposed to be passed on free to the customer.
This is on a volunteer basis only and there is no requirement that they do so.
Secondly when one of the major manufacturers build an install disk for their machines
they usually put in some recognition search to identify the machine. I know Compaq
did this, therefore I assume HP does also. If you try to run those disks they in
all probability will only come back with an argument saying it does not see a Compaq
or what ever type the machine it is made for. In most cases you will need a plane
Jane install upgrade disk or OEM copy to do an install.