I have the exact same problem too. However, the issue only happens when I repeatedly maximize and restore down Windows Photo Viewer. Most people most likely won't notice it unless they constantly restore Windows Photo Viewer's window size.
Some people also say it's a
driver issue. I'm using the latest graphics
driver from Nvidia (on my XFX Geforce 9800GTX Black Edition), but this "glitch" existed since RC of Windows 7. It may also be due to Windows 7 setting your monitor's refresh rate at 59 Hertz, instead of 60 Hertz. There is no way to keeping your monitor's refresh rate at 60 Hertz under Windows 7.