Might be your CD-reader-thingy, I had a old laptop that used to read CD's only once every 5 times I put one in, AND it almost never brought the CD out

so I ended up buying a external one.
Thats one solution, another would be for you to check the online FAQ and maybe send a message to LucasArts on their site, it might be related to a game file on your PC or maybe even your PC itself.