Even better than a C'
data directory: Partition the HD & use only the 2nd partition for all your data. Backing up becomes simpler because you need only copy the entire partition as an image using one of the commonly available disk imaging programs.
The awkward bit is forcing the various Windoze programs to use that partition. Lots of them allow you to specify their storage location, others need a registry hack to do it. There are utilities available that can assist eg TweakUI.