Call external command with relative path

I install gvimPortable in a thumb drive.
The $VIMRUNTIME is d:\Dropbox\Portable\GVimPortable\App\vim\vim73\.
I have a character encoding converter located in d:\Dropbox\Portable
\ConvertZ\ConvertZ.exe
Currently, I use :%!D:\PROGRA~1\ConvertZ\ConvertZ.exe /i:utf8 /o:big5
% to run the converter.
However, when I plug the thumb drive into another computer,
the driver letter of the thumb drive may change to another one.
How do I change the absolute path of convertz.exe to a relative path
referring to $VIMRUNTIME?

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