Use XMLTag(s): WinAPI/Registry
The easiest way is to use the Microsoft XML parser (MSXML - a COM object) included with every recent Windows installation.
See How to utilize XML technology with PowerBuilder article on the mySybase site.
Also check http://www.myjavaserver.com/~unoksoftgroup/xml/xml.html to get a nice user object to handle XML data.
Note that Powerbuilder 9 will be able to handle XML files through datawindow and powerscript.