How do I bring my HP back to factory setting?

Hello,
I have an HP pavilion XT963 that has way to many duplicate windows files on it. I found this out when I downloaded a duplicate finder. Since I dont know enough about what windows are safe to remove and which arent I have left them all on there. I am also having problems downloading some items because it will start the intial download and then I get an error message stating that there's a problem with the internet connection. But its connected and working fine. I am able to get online and get onto any site I want without any problem.
So I figure that if I can reboot the computer to its origional factory condition, maybe it will eliminate all the extra files and be like new. I'd do this by using disc's but I dont have any. So I'm stuck with a computer I can only do so much with. Yet not what I want to be able to use it for. Like downloading school work. I even have issues trying to download adobe reader.