Several things it could be:
Marginal DVD drive
Filesystem corruption on the volume upon which you installed the game
The first and third are easy to check.
Visually inspect the disc for scratches. You can Google methods of dealing with any you find.
Open a commandline window and type "chkdsk <volumeid>: /f /v /r" (without the quotes, and replace <volumeid> with the drive letter you wish to check). Answer the resulting prompt with a "Y" (again, no quotes) and press <enter>. Reboot and let it run to completion, which will take quite a while.
To test for a marginal drive you need to swap in another one.