Bootcamp Windows Partition Won't Boot After Accidentally Changing Boot-up Partition


I was messing around with the startup partition on my Windows 7 partition on my Mac and accidentally changed it to the Mac OS X partition. So when I boot the Windows 7 partition it says the error message "Missing Operating System". This error also shows up when I try to boot the Windows 7 DVD from the Mac drive selector screen. My question is how do change the boot partition on my Windows 7 partition if the partition won't even boot? I really don't want to install a new Windows 7 partition.

Mac Specifications:
Type: Mac Pro (Mid 2012)
Processor: 3.33 GHz 6-Core Intel Xeon
Memory: 32 GB 1333 MHz DDR3 ECC
Graphics: ATI Radeon HD 5770 1024 MB
Software: OS X 10.8.4 (12E55)
Boot to your Win7 installer DVD, and run the Startup Repair option.

You said that you also get "Missing Operating System", even when booting to the installer DVD?
Try that again, but with this method:
insert the win 7 installer DVD, then shut your Mac Pro off. You can hold the power button for 5 seconds - or if you are successfully booted to OS X, you can just shut down normally.
Press and release the power button, and immediately hold the C key on your keyboard. That should force your Mac to boot to the Win 7 DVD. Follow the on-screen tips to get to the Startup Repair choice, and continue with that.