iTunes following wrong path

Hi,

My Mac has decided to give up the ghost. I am currently trying to back up my iPhone to a new windows PC so that I can install everything onto my new iPhone 7 plus.

I spoke to someone on live chat earlier who advised this was easy to do, and it appeared to be however I have run into a brick wall.

My PC has 2 drives, the C drive contains Windows and the games that I play.

My D drive contains everything else.

When I installed iTunes I installed it onto the D drive, however when the back up of the phone started it went to the C drive and after an hour said the C drive was full (its only 100gb)

I've since deleted everything including iTunes.

Can anyone tell me how I make it so that iTunes and all of its data installs onto the D drive, including back ups?