Torrent a Windows disk of the appropriate version form the internet. Then just use the key on the bottom of your notebook to activate it. Download drives for your hardware from HP's website. Good as new, no recovery discs needed.
That doesn't work for replacing an OEM Compaq version of XP (I know, because I just tried it). I used an XP Home disc -- not the one that came with the computer, since that's long gone -- and when it came time to put in the product key it said it was bad (despite it being the one clearly visible on the bottom of the computer). When I called MS they said the key was indeed valid and to call Compaq. When I called them (HP) they said they no longer support the product or the OS and don't sell recovery discs for it anymore. In other words, I'm totally SOL unless I can find an XP version with an activator. It really sucks not to be able to install something legally that you own and are entitled to reinstall when hardware fails. And then MS wonders why people are tempted to use pirated versions. Idiots.