Well if you dont have an anti-virus and you download lot of stuff you could have been infected. Try checking if u have other game that cant be loaded for same reason. If you know what you have on your computer you can always do a quick check in your system folder to see if some file that shouldnt exit are there. If you have an anti-virus or you checked and didnt see anything wrong you can always try to clean your cd and look for scratch. A cd can stop working whit a light scratch or just even dust placed on the right spot. If after cleaning the cd it still doesnt work i would suggest you burn a copy of the cd sometime it work even if the cd is damaged.

Well i hope this helped