BOAC,
I sometimes get this on one of my PCs. The second HD has a bootable Win98 install, so occasionally I find that Win98 has started instead of WinXP!!
As this only ever happens from a cold (and I mean Cold!!) start, I THINK it has to do with the time that the primary master takes to spin up. If the 2nd disk (smaller) reports "ready" before the first, then it boots off it.
A quick "3-fingered salute" and all is sunshine and light.
I haven't bothered to investigate further, as it is a trivial problem - at least for me.
SD