The problem was there with and without the presence of XP SP3.
I didn't scan the restored machine as advised.
I did however install the seemingly unrelated BIOS patch for "more efficient power management" as Sony sneakily put it.
Turns out the efficient power management was a bunch of lousy Vaio's rebooting all the time.
This solved the problem.
<rant>
Sony support can't even call a spade a spade. In my book,
efficient power mananagement != reboot on shutdown
Even on the support call, they are more concerned with my contact details that actually solving the problem. I'm going to puta 50% premium on top of my current rates just for Vaios. They are so bad.
</rant>