wrong system partition

Hello,
I had an OEM copy of Windows 7 on my PC, installed on my C: drive. Last week I've installed a new motherboard so I've had to buy a new copy of Win7. I thought I only had to transfer the license so I ran the setup from the cd. Somehow I couldn't access my old Win7 partition to upgrade it (I was blocked). So I chose the only option left which was reinstalling the system. But in order to do this, so as to not not loose all my data, I chose to install Win7 on my D:. It worked ok, but now my old D: has become my C: and my actual D: is somehow considered to be my system partition. This is confusing because I'm not sure what partition Win7 is using right now and I cannot partition my D: since it is considered as my system. Does anyone know what is going on?

Had to reinstall win7 on my old C: so as to be able to format my old D: Should have remembered that Windows saves older versions in a folder when you install on C:. Just a waste of time.