PC related question. How do I get iTunes to sync/backup/store to my D: rather than C: ?

A couple of options. Do you want your whole library on the other drive or just media? You can split media between drives starting with now, but this will be a big nuisance when you come to move your library in the future.

iTunes: How to move [or copy] your music [library] to a new computer [or another drive] - http://support.apple.com/kb/HT4527

Quick answer if you use iTunes' default preferences settings: Copy the entire iTunes folder (and in doing so all its subfolders and files) intact to the other drive. Open iTunes and immediately hold down the Option (alt) key (shift on Windows), then guide it to the new location of the library.

iTunes for Mac: Moving your iTunes Media folder - http://support.apple.com/kb/HT1449