Rail Simulator Error

I have Rail Simulator, and it has always worked fine, until now. Everytime I try to run the sim, I get this message after about a minute of the computer trying to read the disk:

Does anyone know how I might be able to fix this, or why this message comes up in the first place?
Thanks.
8-)

Does anyone know how I might be able to fix this, or why this message comes up in the first place?
Thanks.
8-)