C.a.R. > Java
C.a.R is a Java program and needs a Java runtime to work. Java is available for all modern platforms. The minimum Java version is 1.4.
The windows installation program will recognize the Java version automatically. If Java is not installed, have a look on the pages of Sun. Go to Downloads, Java 2 Standard Edition, and download the Java Runtime Environment.
Download Java Runtime from Sun
Most Linux distributions contain Java. However, you can use the link above to download a Java version for Linux.
On Mac OSX Java is preinstalled. For older Macs Java is also available.