The official website for The Settlers of Catan

Question about Technical Questions In which case does the installer want to install Java, even if Java is already installed?

Answer

Catan - Cities & Knights is a 32-bit program and needs the 32-bit Java version. If only the 64-bit Java version is installed on a 64-bit operating system like Windows Vista 64-bit, the installer additionally installs the 32-bit Java version. The 64-bit Java version can still be used by 64-bit programs.