Restoring factory settings

Hello everyone, I have a old dell insparon 1500 that I brought back in 2005 that is running windows xp and runs really slow. I already have a new computer, but i was thinking about giving this old one to my little brother and i want to fix it up so he can use it for awhile. I was thinking about restoring it back to factory setting to see if it will run better but i do not have the disk that came with the system. Is there any advice someone can give me on what I should do?