Thanks to all for your replies.
Since my original IDE system disk became too small, about 18 months ago, I have been through two successive IDE replacements bought over the internet, so the idea of a brand new WD SATA disk was attractive, now that it seems difficult get get a reliable IDE disk anymore. Buying another IDE disk may still be the plan of last resort, but I'd like to explore other possibilities first.
I don't understand what "BIOS-compatible" would mean. My experience with the SATA to IDE adapter is that BIOS detects the disk in the adapter (as the IDE primary master), and so does Windows. I had no trouble cloning the previous system disk to it. But Windows will not boot from it, when it is the only disk attached. What actually happens is strange. With only this new disk attached, Windows begins to load, but on reaching the point when you would expect the invite to login, no such invite appears but the process stops and the machine has to be switched off.
Even stranger: if I attach a bootable IDE clone in the secondary master, besides having the new disk attached, the machine boots. If the first boot option is HDD0, the new (SATA) disk is D: (system !!), the IDE clone is C:, and the DVD (secondary slave) is E:. If the first boot option is HDD1, we get the new disk as E:, the IDE clone as C: (system) and the DVD as D:. In either case removing the IDE clone, as I want to do for safe-keeping, prevents booting.
My plan A is still to seek a way to make it boot. In reading up, I'm ignoring (for the present) things like installing SATA drivers, since the adapter is supposed to be hiding the fact that it is SATA. Likewise I think I shouldn't have to upgrade my BIOS, which makes no mention of SATA. The BIOS is Phoenix Award BIOS v6.00PG and the motherboard is MSI KM4M MS-6734 VER:1, of 2003 vintage.
Plan B could be to fit a PCI SATA card and drivers, and dispense with the IDE adapter, but I might be no better off unless someone can assure me that a SATA disk fitted to such a card should be bootable. I've seen reviews of particular cards which said they were not bootable, so I'm sceptical here.
Plan C could be to make a bootable DVD to keep the new disk company in the machine, if it would work the same way as the IDE clone is doing at present. But I don't have installation media for XP, to make such a DVD, and besides it would take the DVD drive out of use, so it doesn't appeal very much.
The adapter comes from a reputable retailer (see
http://www.maplin.co.uk/p/oem-500gb-sata-35-inch-hard-drive-with-ide-conversion-kit-a08ky ) but there is no brand name on the box and no instructions at all, not even how to connect it. On the adapter itself there is JP IDE & SATA BRIDGE and JP103-20330-818-2, if that helps. The hard drive is Western Digital WD5000AAKX. My cloning software is Casper 6.0 (
http://www.fssdev.com/products/casper/ ), which has always made bootable clones before this; it has an option to "repair MBR", which tells me that mine is already valid.
Thanks again for your interest. I would really like to get this fixed!