Which RS version works in Win XP, and where can I get it?

Robot Studio 5.15 was the last version supported with Windows XP Service Pack 3.The reason is that later RobotStudio uses .NET Framework 4.5 which is not supported by XP or Vista. 
You may be able to obtain a copy from your local ABB Service Representative but you will not be able to license the premium version. The license server for that version was shutdown a few years ago.