The single player and multiplayer versions of Unreal 2 cannot be installed on the same computer simultaneously. To install the multiplayer version you first need to uninstall the single player version. During installation, click Options in the lower left corner and check the "eXpanded MultiPlayer (without single player)" option. To play single player later, you will need to uninstall the multiplayer version first. This is a temporary inconvenience and will be fixed as soon as possible.