About treesize.

It's only a tool to sniff out where the big space eaters are. It's then for you to decide if you need to delete something there. Since the decision is yours alone, no one can tell you what to delete since it's YOUR MACHINE.

As to the drive being full, my advice is to delete a few things you have on backup copies then run disk cleanup as well as consider trying CCLEANER (see google)
bob