I'm using a Dell Studio XPS 1640, with an ATI Mobility Radeon HD 3670, driver verion 8.631.0.0

I have a strange problem with Windows 7, every so often I get a problem with my display, when I click maximize on any window, it only fills section of  the screen width.

To be clear the desktop and taskbar does  display correctly. I can also resize the windows to use most the screen, but pressing maximize just displays uses part of the screen. Below  is an example the, the word window on the left is maximised. Note  on latest occurance of the problem, the maximise width is only around a 30% of the screen width, not 60% as in the screen below.

I am using dual monitors, with the main laptop screen being 1920 x 1080 and the second screen being 1280 x 1240, I have the extended screen to left. Problems normally occure when the display second display is accidently disconneted. Note maximise on the the second screen works okay

Some times the problem is solved by a log out, othertimes I've had to reboot, it's a real pain.
Any suggestions?

Thanks