Is this happening when you start the game ? Did you try to right-click on your DVD drive icon and choose open to see if it can be read that way ? Do you maybe have another computer you could try that on ? Can you read other DVDs ok on your drive ?
Thanks you for your thoughtfull answer,
Its simply not reading the CD, so it won't let me start the game.
Since I wrote this, I tried it on another computer and it worked perfectly. Also I'ved other CD's on my machine, and they worked, but not other Sims 3 CDs. It doesn't read them either, so now I think it has something to do with my CD drive and the Sims 3.
Rather odd isn't it?
See if this link helps you: http://sims3.crinrict.com/en/2011/03/error-cyclic-redundancy-check.html
Easiest is probably to just install the download version from Origin instead. It's free if you have the DVD.
