Starting from version 0.0.21 rxtx as well as Java comm is included in every release. So there is no need to perform the instructions on this page, unless you know what you are doing.
Its very easy to install RXTX, first you need to download the latest binaries from http://www.rxtx.org in the downloads section you will find the 2.1 release with the following file convention:
The (Final) version is the latest stable build.
Open this Zip file and:
copy rxtxSerial.dll into your <JRE>\bin\ folder and then copy RXTXcomm.jar into your <JRE>\lib\ext\ folder.
Where <JRE> could be: For Windows XP [Spanish]: c:\archivos de programa\ For Windows XP [English]: c:\program files\
Follow the instructions from this page: http://wass.homelinux.net/howtos/Comm_How-To.shtml