Windows 7: second monitor issue

Im having a problem when using a second monitor. I recently built a new pc in which I am running 2 nvidia GTX 460 cards under sli. For some reason every so often the mouse of its own accord flicks from monitor 1 to monitor 2. This usually drops what ever is on display on number 1 to the menubar. There is no predicable pattern to it as some times I can go several hours with nothing happening - and at other times it can occur every few seconds. This is not so bad when using stuff like a word processor but is a big issue when playing games or a graphic program for instance.
Ive tried running the pc with only 1 graphic card connected and it does the same as with 2. I doubt that it is the monitors as they are both new - one being connected via hd the second (a touch screen) being connected via standard vga socket.
Anyone have any ideas on how to stop this?



Thanks
Antony

edit - I should mention that I get this happening whether I am running sli or not.