Parental Controls on my PC

Well, we have started letting our oldest child use the computer to visit sites like nickjr.com or disney.com. I think it's good that he gets comfortable using a pc to help him in the future, however I would like to be able to let him play the games without me in the room. I know it's easy to get a few stray clicks and end up on a site that is inappropriate for younger audiences. I am open to any suggestions on Parental Control software that anyone out there has used. I would love to find a free one, but I understand that you get what you pay for, so anything as long as it's reliable and customizable per profile on the pc.