The computer is a very good device that can store as much as you let it and as much as the hard disk can accommodate. That includes all kinds of logs, all the sites you have been to while connected to the internet and even all the virii that entered your computer without you knowing. Without any virus, the temporary internet files alone can choke your computer and bring it to crawling instead of running.
That is why it is important to clean up computer every now and then. Regularly. Especially if you have Windows as your operating system. You need to overhaul.