How to take off all this unknown memory on my computer?

Hi I was wondering if there was a way to taking all this unknown space off my computer. My computer holds 74.4GB of space and I have only 9.86 GB left. I know im not using now even nearly close to 40GB of space so idk how I am using over 60GB of space. I cleaned, defrag, deleted stuff in my TEMP folder, and i'm to scared to go around and delete stuff knowing I might fck my computer up. The only problem I can think of is when I switched from xp to vista. All my xp stuff got transfered because idk what was on my computer a virus or something but it wasn't acting right so I installed vista on my computer and it seemed to be all better exepct now I got AALLLL this space on my computer used. If anyone could tell me what I can do and what is going to make my computer have more memory because this is ridiculous.