Well I've been through all my nvidia settings and nothing changes the behaviour. I've had nview on and off. I found the setting:

>> Enable/disable window from spanning across displays

but this just stops windows from going onto the second monitor if off or allows them to be dragged if on. I have no problem dragging the window to the second screen.

My only problem comes if I lock the computer - either with 'Windows Key + L' or when the screensaver kicks in.

Screensaver with no lock/password keeps the window where it should be, partially over onto the second screen. On the other hand, panels/palletes from Photoshop, Fireworks and Modo stay where they should be.

So I'm stumped. It's like the system treats a locked system as different from an unlocked one and wants to put the windows back on the monitor they were opened on. Very strange but I suspect is a quirk of my system. Perhaps an update to the drivers will help - but I'll leave that for another day :-)

Thanks for all the suggestions folks.