That's easy - click on 'run' and type msconfig in the box. press enter. On the system configuration window that pops up, click the BOOT.INI tab. The third line of that tells you the default OS which starts. It might say
default=multi(0)disk(0)rdisk(0)partition(2)\WINDOWS
Then under [operating systems]
comes a list of all the OS's known about at boot time. The lines tell you which partition each is installed upon. In my case, referring to the default line, it says
multi(0)disk(0)rdisk(0)partition(2)\WINDOWS="Microsoft Windows XP professional" /noexecute=optin /fastdetect
You will have a second line, with a different partition number and (probably) different description.
Now the partition numbers start from ONE, so that is the first physical partition on the drive. Partiton(2) will be the second and so on. In many cases, the first physical partiton will be a hidden OEM 'restore' partition, and the actual OS partition will be the second, or partition(2). Just to confuse the issue, drives are numbered from zero, and logical and primary partitions are numbered differently, but I'm only warning you in case your situation is not as straightforward as it should be.
You will now be able to tell which OS is where, and by looking here
http://support.microsoft.com/kb/289022, remove the one you don't want. Finally, you could format the unused partition and use it for OS imaging and backup.
As you see from my own example, it matters not one jot which partition the OS is actually on, there is no need to even dream of moving it to another partition. The main difficulty you could conceivably have is if the OS you want to keep refers to itself as being on drive D: Lets us know if that is the case, and if you desperately want to do anything about it, the simple answer is you cannot. see here
https://www.techspot.com/vb/showthread.php?p=747557#post747557