You can download and install a version of XAMPP that contains PHP 5.1.6 but it is not possible to change the PHP version in what ever version you are using at present.
By using a different name folder for your XAMPP version that includes PHP 5.1.6 that you install then it is possible to have 2 versions of XAMPP on the one PC but only one can be started at a time.
This is the method used by many and even I use it for the 6 testing XAMPP versions I have installed on my PC on various drives and partitions.
Older versions can be found at SourceForge here:http://sourceforge.net/projects/xampp/files/XAMPP Windows/
Does this help?