Just in case any one else is as stupid as me.....
I've just updated the BIOS on my gigabyte P35C DS3R (apparently the ram voltage was a bit screwy in the old one) and after updating, you're advised to reload the "optimized" defaults.
the big problem with that is that it turns OFF the RAID setting. When you then boot into an xp partition (it's my safety/comfort OS, on a separate HDD) without putting RAID to ON, it kills your RAID 0 partition and your vista install with it .
Still, the RAID 1 partition on the same set of drives survived (albeit degraded) so no real harm done. If nothing else, it's forcefully reminded me that RAID 0 is only any good for stuff you really don't care about.
Now, must remember to ghost my vista install this time round