Why is my computer so slow all of a sudden?

It's crawling at the moment. Even starting up takes a good few minutes more compared to normal. Not sure why it's suddenly doing this.

I've run CCleaner. I'm trying to run an avast! scan but when I try and do a full scan it just stays at 0% for about an hour before I give up and stop it. Currently running the Disk Defragmenter but that's stuck at 9% for the past hour or so.

I also cleaned out any dust from the computer just in case that was causing the problem.


What the hell could it be?