As someone else said - installing SQL Server to a non-root drive still puts a LOT of crap in the root!! I would do one of two things: look for things you can delete from C to give you more size (all those blue windows update folders, temp directory crap, other install packages, etc). NOTE: removing some things can be problematic when other updates need to be installed!! I actually copy those folders elsewhere then I move them back if necessary later.

The other option is to get a tool like Paragon's partition manager to resize the boot partition if you are lucky enough that they didn't make it the full size of the disk and there is (or you can create) some free space.