I bought a Microsoft Laser Desktop 7000 keyboard and mouse a few weeks ago.  I inserted the included rechargeable battery straight away and used it until it ran out of charge.  The trouble now is no matter how long I put the mouse on the charger, a message pops up saying the mouse battery is critically low.

When I put the mouse on the charger, the red led on the mouse flashes so I assume it is charging.  I've tried putting an Energizer 900mah battery in there, but the same problem occurs.  Any ideas?