Sims 3 not working on my computer, it has windows 7.

[samar394]

I have downloaded the Sims 3 online but when I open it,it says that it can not detect the CD and needs to shut down. I had been playing the game for a year or so and had uninstalled it a month ago and now its reinstalled and the problem has started since then. There is no problem with the launcher, this error msg comes when the game has fully loaded. Since I don't have a disk as I have downloaded it from the internet, why is it asking for one when previously it wasn't ? Thanks in advance !!!