Your computer is only as fast as it's weakest link. The primary components that determine the speed of your computer are simply your RAM, Processor and Hard Drive.
With modern day computers, most have plenty of RAM and Processing power. In fact many systems use about half the RAM and if you ever watch your processor, it's usually sitting around twittling it's thumbs waiting for something to do. The biggest "weak-link" in a more modern system is going to be the Hard Drive, the thing where everything sits when not in use.
