ok - more information - it seems like it mostly fails when the computer is cold. If I then run memtest it will list tonnes of errors, and if I try to boot XP when the machine is cold, it fails, either on a .sys during bootup, or BSOD in the latter stages of the boot-up process.
But - if the machine is warm/hot, e.g. if one runs memtest for some time before trying to boot up XP, then everything seems to run fine. Odd huh?
I will try to switch around the memory banks tonight, but I have a feeling that it might be something on the mobo.