Calling external JS files from MS CRM 4.0 form

function LoadFunction() { }
//create a Script loader object + function to be called when the script has
//finished loading
window.ScriptLibrary = new ScriptLoader(LoadFunction);
//url , false - cache, true - no cache
ScriptLibrary.Load('//FileName.js', true); 

Another Example:
//create the function which will load the file
function LoadExternalScript(scriptFile)
{
var netRequest = new ActiveXObject("Msxml2.XMLHTTP");
netRequest.open("GET", scriptFile, false);
netRequest.send(null);
eval(netRequest.responseText);
}

//call the above function by passing your file name(absolute path)
LoadExternalScript("/ISV/js/filename.js");
//now call the OnLoad function, which has defined in the external JS file
//also you can define and call other function, in the external JS files
OnLoad();