HP Desktop logs in and then logs off

Hello,

I was wondering how I can reset my Hp Pavilion desktop with out a CD. Everytime I try logging in to my account or any other account on my desktop, it won't allow me to. It says logging on and it says saving files and takes me back to the page where all the accounts on my computer are. If you can please help me how to reset my desktop with out a cd that would be helpful. I also was wondering if I can still have my files saved on my computer.
I'm only 14 years old and I want an easy way to fix it, thank you.