Sims 3 worked once but not again

Hello,
i bought simsm 3 soon as it came out for my laptop. I have windows vista.
I installed the game, everything was fine, i played it, but when i took the disk out to finish playing and put it in, when i wanted to play it again, it didnt work. So i uninstalled it, then instaled it again, played it again, all was fine untill again, i took the disk out when i was finished playing, put in next day to play it again, but didnt work....why is this??

Thanks