Please help with 'full' virtual drive

Hi.  I'm using Windows 7 in a 40gb virtual drive on a MacPro (Fusion 5).  The virtual drive is full, sluggish, and can't be backed up because there is no room for the necessary temporary files, I guess.  I've tried deleting document files (5 gb yesterday) using Windows, but was only able to recover 1 gb on the virtual drive.  How can I find out where Fusion is using all the disk space so that I can get the files to a workable size?

Thanks for your help.  I've looked here, but haven't yet come up with an answer for someone else.