your "user" files/folders, such as desktop files, document files, music, pictures, videos, etc... tend to be placed on C drive. many programs like to automatically create personalized folders and place files into those folders, which again would all be on c drive.
ugly and painful, right click and goto properties for folders on C drive. to see how much a give folder is taken up space, keep doing it till you find a program or videos, or pictures or what not that is eating up the space.
you can also do a "search" via windows explorer on c drive and let it hit all the files, and then reorder files based on "size" to see the larger files, then look at folder paths to see were these files are located for the larger size files. this is...