Dude. I hope you have found the solution, it has been pissing me off for a while.
however if you havent I have just sorted mine.
Control Panel > Appearance and Personalization > Personalization > Display Settings
then click on what ever screen is too big if this is monitor 1 it has a big "1" on it, monitor 2 with a "2" etc.. click which ever monitor square is bigger and reduce the resolution on the slidey bar so the screen match sizes.
If it just one screen I assume its trial and error. My laptop monitor is 1366 by 768 pixels and my 2nd monitor was 1920 by 1080 pixels so yeah just match them.
(I have windows vista by the way)
Hope that helps!