^^^ Not an expert either, so I think this is right in saying that having little left on your HD can result in things being slower as it takes longer for your computer to search through all the files. It's always good to leave quite a bit free just in-case. I'd suggest getting a bigger HD as most nowadays come in 500GB to 750GB usually.
There is an option to reformat your PC aswell but that would mean starting from nothing. All your data is gone. Google for more info and instructions.