If you have programs installed to D: they won't likely work when you reinstall Windows, so you'd have to reinstall theme (delete any D/E folders first). STEAM is an exception, as there's a method where you just keep the "Steamapps" main folder and steam.exe so you don't have to re-download games.
You probably want to backup the "Documents" folder for example to another drive, and write down program names, export e-mail etc to make sure you have everything. Think it through carefully.