Nautilus is the file manager like windows explorer in Ubuntu - it comes up as 'places'.
I don't use Xubuntu as much so I can't remember the equivalent, but it's on the menu (right-click on the desktop if you can't see this?) and is similarly easy to use.
It probably won't call the partition the D: drive, but it will probably be in the list as 'file system', and should 'mount' (let you go into it) when you browse to it.
If you're not planning to dual-boot, when you install you will be nicely allowed to repartition the hard drive and it will give you the default option to pick if you're confused...
Hope that helps!