Got it sorted now. It was actually easy to integrate the SATA drivers into an XP bootable cd-image using nLite, and from that point everything went smoothly.
Now typing this from XP, and god it's so much quicker. Boot up is a lot quicker, but it's not just boot-up, XP doesn't sit there for 2 mins after boot-up chugging away on the HDD for no reason like Vista did. Part of the HDD was also took up with a hidden recovery partition, which I've deleted so I also have the full 160GB available now in a single partition. All drivers seem to be working spot on - of course it's vital to check drivers are available for your laptop hardware before doing this.
Disc performance seems quicker, everything opens quicker and runs better. Wireless network now connects within a few seconds of the system booting rather than several minutes under Vista (and half the time it didn't used to connect at all). As for the other benefits (I'm hoping some games which are listed as working with intel 945GM but didn't work under Vista, will work under XP) I'll only be able to answer that once I've tried them. Other things like battery life, again too early to say.
Very happy to be free of Vista though. I'll post back more about benefits/drawbacks after a longer period to assess things.
edit: Half-Life 2 works under XP on 945GM whereas it would crash under Vista. Sims 2 much better framerate. Cool
2nd edit:HL2 does still crash, but not until the 2nd load (after jumping through the window). Might be fixable, I dunno. Don't actually want to play it on my laptop (see sig) so I won't be investing time in finding out. FYI under Vista it crashed a few seconds after a new game was started.