So.. recently, the HDD in my computer has been decreasing slowly even though I didn't really install anything. I scanned my computer thoroughly with a Norton 360 and nothing was found.. What seems to be the problem o.O?

Have you been saving files, pictures, music files, video files etc to the HDD lately? Those, especially video files, take up a lot of space and can bog down a computer.

My suggestion is to maybe weed out a few things and see what happens. Not saying that this is exactly the issue here but it worked great for my computer!

Are all your drivers up to date? How much room is left on your hard drive? How large is it?
Also doing a defrag if you haven't done one in awhile may help to speed things up.

Thanks for the suggestions, I think I might be adding a few files here and there so it looks like the HDD is slowly decreasing. Doubt it's a virus xD