How do i reset my computer to original state

Hello, I have a HP compaq 6910p and I bought it from someone and when I got it and started it up it had windows 7 on it and I thought cool. After owning it a month windows asked me to validate it, and I did, and I found out it is a fake copy, so now there is a little note at the bottom of my screen that says this is not a real copy of windows. After looking at bottom of my computer I of course saw sticker that says windows vista and key code for vista, why I never looked before I will never know. I of course have no disks or anything else with computer so I need to remove fake windows 7 and return computer to its original factory settings or windows vista. Can anyone help me?