Can I put password in my web browser?

I have Magic Jack which means that I need to leave my computer on for a long period of time. But I have a guy that always come to my house (even in my absence) to use my computer. I want to know if there is a program that will block my web browser when I want to without interfering with my Magic Jack; or is it possible to put password in Explorer or Firefox?
Any comment will be appreciated.

Kliford