We are aware of an issue when installing HITMAN™ from your PC Disc. There is a known issue with Disc#1, you should have received a replacement Disc#1 at retail. If you did not, you can follow these simple steps to install HITMAN™ onto your PC.
1) Please insert Disc#1 and enter the key from the leaflet as instructed.
2) After your Steam code is displayed, please download the following file and run it: http://cdn.sqexeu.com/files/documents/DiskInstallHelp.zip
3) Follow the instructions on the screen to finish the installation
We hope you will enjoy the game and apologise for the inconvenience.