1) wrap that setup.exe in a .bat file, and use ~dp0\
2) wrap that setup.exe command in a vbs script and use a variable to
reference "the directory in which this .vbs exists" (that's what I do... I'm
a vbscript person)
3) I think you can also put config.xml in the same folder as setup.exe, and
then no reference the folder at all--just put setup.exe /config config.xml
as the command line. Test that tho. But I think that works.