Ok, I see. Anyway, from info I found here and there, the function you were talking about is apparently not supported on newer BIOS versions...
The other solution (but be warned, severe headache probability...) would be to find the correct XP SATA drivers from the chipset's manufacturer site and integrate it to your XP Install disk. This is described in "Stand Alone Windows Installation" section (on the same page you linked to). By the way, by "processor", I guess they mean chipset, on that page... And "native-ATA", not "native-ATI"...
Basically, it's the same procedure as installing a pre-SP1/2 XP version on a desktop PC with a SATA HDD. It's called an F6 SATA/RAID AHCI driver, because normally you need to press F6 right after booting from the CD and loading the drivers from a floppy. Except that, here, you probably won't be able to use a floppy... So you need to integrate those drivers directly in your XP installation.
Actually, if your XP version has SP1/2 integrated, this driver may not be required, since these later versions are SATA ready.
Instead of the tools they're using, I'd suggest using 7zip or winrar to extract the files from the package (depending on package, it should work).
For integrating the extracted drivers, it's easier with nlite
and it'll create the bootable ISO at the end. They have nice and clear tutorials too, if you need one (look in "guides" section).
To determine what SATA controller you have, you can use Everest Ultimate
Here's a forum topic I came across that may help you too. link
Just make sure you have exactly the same brand/model chipset before downloading anything from there. And I strongly suggest you to try your SATA controller's manufacturer site first.
Honestly, I wouldn't advise you to downgrade to XP. First, as I mentioned earlier, keep a tube of aspirin at hand - problems are to be expected: "can't find device", "process failed" messages or even BSODs...
Second, nowadays a lot of laptops are expressly designed for Vista, so even if you install it successfully, XP is not guaranteed to work correctly, if at all...
If you're still willing to try, I strongly suggest you backup your existing Vista installation with some disk/partition imaging software. Acronis has a good reputation or here
are some free ones as well. If your laptop came with some recovery CD or partition, these may be a substitute to the previously mentioned method.
Well, if you still have questions, feel free to ask (and I have the feeling you'll have such