Why is my computer slow?

Because you might be a slob.

Seriously, I've been in a computer ever since 1984 and I've seen my fair share of computer problems solved by simply cleaning out the case. Twice a year shut down your computer, unplug all cables, take the tower outside and use a compressed air can to blow out all the dust. Seems like a simple thing or that your case isn't very dusty but I've seen people throw out perfectly good computers because they thought theirs was "bad". I blow out the dust and it fires right up and works great. So don't let this happen to you.

More info on the influence of dust in a computer and the damage it can cause.

BTW you read it correctly, I've been in a computer since 1984.