Something is locking Vista

I meaning locking the desktop, as if I pressed the Windows key + L, not crashing. I then have to enter the password to get back to what I was doing at the time - which is very annoying!

It started yesterday and happens every minute or so but is quite random. It happens no matter what I am doing. Once it happened twice in 5 seconds. I tried turning off the screensaver - it wasn't that. Rebooting twice didn't fix it. I did a Google search but nothing came up apar from the screensaver and power options settings which I've already checked. Something must be locking the desktop - any ideas?

Random guess, but try a different keyboard?

That's not a bad idea to remember. As it happens, last night it stopped as suddenly as it began. I shall bear your idea in mind though if it returns.