How do i find my product key have lost it?

i needed to re install my windows xp home addition, i put the disc in it started to install then when it said entre product key it wasnt on the side of my tower and i cant find it any where now i cant use my computer untill i entre product key and i cant cancel the insterlation as it keeps going back to entre product key, luckylu i have a laptop as well but i still need my main pc plz some help me