messenger scripting questions

Hello

I have Windows Messenger and MSN Messenger installed on my computer.
I'd like to initiate a chat session from a jscipt. The following script
works fine on my computer and will open a chat session using MSN:

start.js:
var obj = new ActiveXObject("Messenger.UIAutomation.1");
obj.AutoSignin();
obj.InstantMessage( "user@xxxxxxxx" );

When I try the same script on a cumputer that only has Windows
Messenger installed nothing will happen.

How can I know which messengers that are instaleld?
How can I know which is the default messenger?
How can I force that Windows Messenger is used if the system has both?
Why does the above script fial on systems that only have Windows
Messenger but not MSN Messenger?

.