After using the Steam disc installation method, I get asked to insert a second disc which is not included. What can I do about this?

Due to a slight error in the installer logic, the provided steam install disc that comes with the Retail PC Box version of the game could prompt for a second disc. This is a bug and not an indicator for a missing disc. To fix this bug, just cancel the request for this disc and the Steam client will continue downloading the rest of the game directly from steam. Afterwards you can just start playing as normal.