C++ system function-Collection of common programming errors

You need to enclose the path in quotes as it contains spaces. As normal double quotes would end the C++ string literal, you have to escape them:

system("start \"C:\\Program Files (x86)\\Tibia\\Tibia.exe\"");

A simple system("C:\\Program Files (x86)\\Tibia\\Tibia.exe") would likely do the job as well, as this is what the start verb does.