User @Jos gave already a good answer in his comment.
Just to clarify it further: it's true that Ubuntu has somewhat different partitioning model than Windows and thus doesn't use letters for partitions.
But you can freely place additional partitions (mount it) under /media/ directory - but of course, please reserve at least one partition for the Ubuntu needs (or alternatively two):
- one for Ubuntu itself under
/ - equivalent of C: in Win (20 or 30 GB would be enough)
- and the second for users directories
/home - equivalent of D: (let's say 200 GB).
All remaining partitions you can place under /media/ directory. Sth like:
/media/e
/media/f
/media/g
You can do it during the installation by choosing Something else, or even later, after the installation, with the mentioned GParted program.