Thanks everyone, I got it sorted now and avoided needing the recovery install. Not sure what the problem with recovery install was, I was going to try again with as many drivers as possible unloaded or set to generic and my AV uninstalled etc, but in the end I bypassed the need all together. I was thinking of moving to Windows 7 with a clean install at my next major H/W upgrade anyway, so shouldn’t need any more recovery installs anyway.
Thanks to czr for alerting me to the fact that a simple controller swap was all that's needed. Honestly I was very poorly informed about the whole AHCI on Win XP thing before I started. I'd googled it and read a lot of stuff at various random sites, much of which turned out to be total rubbish parroted from one person to the next. Things like "you need to re-install windows every time you change between IDE mode and AHCI" (either way) in bios, and of course the endlessly repeated "you'll get no performance improvement anyway, that ncq is only for heavy server loads and does nothing on a desktop computer". No wonder I was confused after reading so much junk. I really thought the darn re-install was some type of essential process in making AHCI work when clearly it isn't.
BTW. In the end I was going with my "migrate back to IDE, load sata drivers, migrate back to sata" idea, which I'm now 100% certain would have worked, but half way through the process I realized that I already effectively had two sata controllers (built-in) anyway. I suddenly noticed that my bios had two relevant settings,
1. "SATA MODE", which could be set to : "IDE", "raid" or "AHCI". and
2. "SATA 4/5 as IDE", which could be true/false when the main sata mode was AHCI.
So suddenly it was obvious that the second setting would let me do czr's two controller trick, all on the one controller! I just set it to AHCI (1st setting) but with SATA 4/5 still IDE (second setting) and then popped the sata cable over from SATA 0 to SATA 4. Booted it (still in IDE mode but AHCI now active in bios so it would prompt for driver once XP had loaded), loaded the driver, turned off, popped the sata cable back to sata 0 and I was done. Would have been 5 minutes tops if I had known what I was doing from the start, too bad I wasted a whole day beforehand. Anyway that's a handy trick to keep in mind, I see a lot of mainboards now days have that split SATA/IDE option that saved the day for me.