You could have a Virtual Disk tucked away in a folder within a folder from a training course two years ago but it's not always so easy to find!
Now, you could go old skool open up a cmd box and type > dir /s. This will list every single folder and the total file size.
However, unless you have very folders (unlikely) and very few files (more unlikely) this is difficult to read.
Even, dir /s /p won't get you that far!
What you need is what Windows Explorer should give you but doesn't. You need to be able to see all your folders visually and see who's hogging the juice.
There are many tools out on the web that will help out here. I recommend WinDirStat. It's free, simple to use and presents everything you need to know after quickly scanning your disk.