setInterval(requestQueue.callQueue, 100);
Peels off the callQueue method from requestQueue and passes it to setInterval. Consequently callQueue has no reference to requestQueue; when it is called back, this will be set to window not requestQueue so the attempt to evaluate this.callToArray.length will cause an error.
this in JavaScript does not work in the way you would expect from any other language. See this question for an explanation. If you only have one requestQueue instance you can just reference that instead of using this, otherwise you'll need to look into function binding or a closure.
Incidentally I'd not recommend UA-string-sniffing for detecting the right XHR object to use. Try feature-sniffing instead:
var r= window.XMLHttpRequest? new XMLHttpRequest() : new ActiveXObject('Microsoft.XMLHTTP');
(this would also use native XMLHttp on IE7+ where enabled, potentially avoiding an ActiveX prompt.)