Enter BIOS setup and see if there is an EFI boot disk set first to boot, remove it and change your HD (or DVD drive) to boot first.
This should allow an MBR HD to boot however it is never a sure thing that Win7 will start once you change such a major hardware component. The only way to assure this is to SysPrep to move HD to another computer,
run Paragon Adaptive Restore CD on the new hardware, or use Acronis True Image 2010+ with Universal Restore to image it over while making the necessary adjustments.
If you cannot remove EFI from the boot options or change it in other BIOS settings, then you'll need to install following these special steps: UEFI (Unified Extensible Firmware Interface) - Install Windows 7 with - Windows 7 Forums