Lo dudes.

Hope this helps someone.

XP Pro won't find your Picture's even if you've show it the correct folder IF YOUR CPU IS OVERLOADED

Just running CPU Burn in on a Quad Core, and the screen saver has popped up with the No Pictures Found message, which was weird, as it's always worked ok.

Switched off one of the 4 CPU Burn In processes, and screen saver works again.

Replicated multiple times, so clearly needs lots of CPU cycles spare to make it work, or the SATA controller gives up

As I said... hope it helps someone one day.