If your Norton is up to date and properly configured, odds are you're fine. Its just a consequence of using a computer that files build up - that's the way it goes. You can do a manual cleanup of unnecessary files, but here are a couple simple-to-use, free apps that will do it for you.
Cleanup4! - Small download, small footprint, super easy to use, pretty thorough, not many features.
CCleaner - More powerful, a little more nit-picky to set up and use, but still easy - does lotsa stuff - read and understand the documentation before doing anything with it other than cleanups.