I've just had a mammoth "session" getting Windows XP to install onto an HP DV5-1209ax. And it worked! Yeeha! So the reason for this post is to let you know that it is doable, even though MS and HP have made it very hard to do so. If anyone has need of finding out how it's done, let me know, and I'll post the whole drama. Since the 1209ax is a very specific model, what worked on this model may not work on other "specific" variants.
As a teaser, the main things that were involved to make it all happen were these:
1) Find RAID SATA driver for the M770 chipset. None avail per se, so had to use Gigabyte equivalent.
2) Slip-stream the RAID SATA driver into an XP Pro installation and make a new boot CD.
3) Find the right XP-compatible driver for the Realtek RTL8168/8111 LAN to work
4) Use Mobility Modder from DriverHaven.net to get the ATi 3650 discrete graphics to work properly
5) Use DriverAgent (I have a subscription) to find all other drivers
All up, including the conversations with HP (who were VERY helpful, for a change), it took around 3 hours. Laptop certainly is markedly faster now that it has the Vista bloat-ware off it!
I hope this is useful for someone. Contact me by either leaving your email address of sending me a message through the Daniweb system if you need specifics.