When connecting an external 1080p monitor (Samsung SyncMaster P2470LHD) via an HDMI cable I was getting an underscan (a black border around the image) on the screen when running Windows 7 in Bootcamp. Since there is no slider to adjust underscan as there is in OS X a different solution was required.
The ATI (now AMD) Catalyst Control Center (CCC) is required to enable the automatic underscan applied to external monitors. Details of how to install the ATI CCC on a MBP running Win 7 in Bootcamp and replace the Apple Video Drivers can be found on the Apple Discussion Forums.
The lastest ATI CCC drivers – http://www2.ati.com/DRIVERS/mobile/11-10_mobility_vista_win7_64_dd_ccc_ocl.exe
