a) [even though it is not necessary] before OS installation you might want to remove everything off your HD, including formatting info ~ that part can be done with Win98 boot disk and FDISK utility
b) XP start-up disks [a whole set] are available for download on Microsoft support site; also search this forum for thread on that and direct link
c) HP doesn't actually make HDs; if anything they will stick their label on it, so I would write down model number and serch the web based on that. Once you have the OEM manufacturer you can use links on this site to get the utilities.
d) even though you bought this PC second hand, you should be still able to get some support from HP; if you have to go and re-register it with your name drop the purchase part, and rather go with 'it was given to me by my older brother' story.
a) a quick note on XP start up disks - sets for Home and Pro are different, so make sure that you have a right one.
b) formatting can be used for quick removal of pre-existing data, however many users have found that wiping the HD with manufacturer's utilities solves some headaches that might show-up later
c) FDISK command utility can be used to remove any and all formatting info ~ so from logical structure point of view your disk returns to out-of-the-box state. Manufacturer's utility disk will do the same.
Fdisk utility is usually included on majority of boot disks. Check BootDisk.com
. For usage do a search on the web - plenty of info there