This is just a guess. because I do not have the code to try it out, but I would try this:
string $startExe = "system(\"start c:\\xxx\\xxx.exe\")";
menuItem -label "start Client" -command $startExe;
or if that does not work you might try:
string $startExe = "system(start c:\\xxx\\xxx.exe)";
menuItem -label "start Client" -command $startExe;
The backslash does not look right, it may have worked, but it looks more correct in the last example I listed. It may not even be needed.
Working with strings is a lot of fun . . . :headbang: Best bet when you get into trouble with ANY Maya commands is refer to the reference library or look online for example code.
"The Sage as an Astronomer: If you still see the stars as something above you, you lack the eye of knowledge." Friedrich Nietzsche