Windows 7: Logon Screen - Flashing and horizontal lines

Hi all

I have got a wierd problem, Sometimes when I get to the logon screen, either via startup or when I have been away from the pc I get the logon screen, but its half the size and flashing and theres horizontal lines. I then have to reset the machine and carry on.

Now I would think its the graphics card (Nvidia 9600gt) but this only happens on the logon screen. Playing (Wow, css, mass effect 2) there are no problems. I have also noticed it only happens when I have been away from the pc for a while.

If I boot the pc and come back a while later or when then screensaver has kicked in and I get prompted with the logon screen again. I have tried to change the screensaver but it didnt solve that.

Any ideas please guys.