Geeksquad is your first problem. The people that work there are smart for the most part, but their policy is to rob you shitless before ever actually helping you.
Right click on C, go to properties. First run a disk cleanup, when that is done go to the Tools tab and defragment.
Defragmenting doesn't technically give you more space as much as it moves fragments that were taking up extra space. You can get some reasonable gains from it.