Not sure why it would be changed to D drive 
It all started when I built a new 6-core AMD ... MS wouldnt let me reuse a WIN7 upgrade DVD I'd got 3 years ago to upgrade from VISTA.
I tried to do a clean install of WIN 7 from that DVD onto new PC's new SSD, and it wouldnt recognise the Product Key, so I couldnt Activate it ...
... so I started again by installing the old VISTA, and then used that WIN 7 DVD on that base to take it up to WIN 7 ..... worked fine, but had left VISTA on 'C', and WIN 7 became 'D'
So now I didnt build an ISO to do a clean install of WIN 10, by installing it on top of the WIN 7, its kept D as SysRes.....
I'll have a fiddle later - its not an issue, just looks odd after so many years of 'C' ...