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

#1
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)
 
#2
Boot to your Win7 installer DVD, and run the Startup Repair option.
http://windows.microsoft.com/en-us/...ns#what-are-system-recovery-options=windows-7

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.
 
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