Windows could not start because the following file is missing
or corrupt:
<Windows root>\system32\ntoskrnl.exe.
Please re-install a copy of the above file

I'm on the same computer using Windows XP off a diffrent HDD, how do I fix this?
The OS I want is Windows 7, it came from a computer that's motherboard broke.

YOu would need to boot with Windows 7 media and run a repair on the win7 installation.

Or if and you have a Windows 7 installation disk, Run Ubuntu off a USB, format the hard drive with windows 7/xp on it, insert the windows 7 disk, re install it.