I have successfully installed Windows XP SP2 32-bit (then upgraded to SP3) on a HP Pavilion DV4 1275mx which originally came with Windows Vista 64-bit. (furthermore.. I have a Vista64/XP32/Ubuntu32 tri-boot.)
I have ALL drivers working - video, sound, wired network, wireless network, SD card reader, bluetooth, and accelerometer (protects against HD being damaged when dropped).
Note: The only one that is NOT loaded is the ACPI driver which handles sleep/hibernation/screen dimming/fan speed adjustment. If anyone has a solution to that please let me know. Its actually a fairly minor issue.
I cannot post a quick 'do it this way'. I've hacked and modified drivers from other manufacturers and OSes to create working ones for the DV4, which I'm very sure I can't post due to copyrights and EULAs. THESE ARE NOT AVAILABLE ANYWHERE ON THE WEB. At the risk of sounding like a tech support douche, I highly recommend sticking with Vista.
If you're set on converting to XP, I can make it happen, but I need to see some interest before I take the time to post a VERY lengthy tutorial on how to get all these drivers together and hacked to work. Reply if you want it. If I get enough interest, I'll write it up.