HP is your main OS support as that's the deal HP made with MS in order to provide OS with the laptop at lower cost. XP is now too old to be sold by HP(out of stock) or general retail sales. If you want XP, then you need to check eBay or Craigslist for some vendor you trust or any "used" source. Since, this is a laptop you may get lucky and have a "recovery/restore" disc made for your laptop. There are legal vendors offering such, just enter your HP laptop model# and see if they have or make it. Also, retail versions of OS will install too, you will then need proper drivers to finish the install 100%. Some vendors offer unopened HP restore discs, even upgrades(if applicable) that work with your HP key, worth considering.
http://www.restoredisks.com -OR- any similar vendors