Wscript.shell Run And MS Office Files

My work setup is Vista Enterprise 32 bit with SP1 and Office 2007 and IE7. I am writing a browser based application using Visual Studio 2008. I have the following piece of jscript code:

function runFile(_fileName) {
var objO = new ActiveXObject('WScript.Shell');
objO.run(_fileName, 1, false);

var objO = null;
}

and it works just fine for various file types (HTM, MSG, PDF, TIF and TXT) where it opens the application that has been associated with the relevant file extension and displays the file. But when the code is passed a .DOC or .XLS file name a copy of the relevant applicaton instance is started but before the document is displayed the jscript code gives the error message 'Unknown exception' with no further details. why the .DOC and .XLS files are not opening as expected?