At What Point Is Global.asa Run?

I'm wondering something about global.asa. We have a stats package that has been written for us. It tracks sessions by tracking session IDs rather than IP addresses, because this is much more accurate (tracking by IP can only give you an average guess at numbers of sessions).

The problem we have is that static HTML pages do not create a session ID. It remains as "-" in the log. I'm wondering if we can use a global.asa file to force IIS to open a session? The thing is, this will still be useless if global.asa only runs when the ASP file is requested, rather than when the first site file is requested.

So, now you have the background, the question is quite simple:

Does global.asa run when you first hit a site (ie: when default.htm is called) or does it run when you first hit an ASP page (like search.asp)?

Obviously, in the latter case that can mean that it's perfectly feasible for someone to spend half an hour on a site and never actually fire the global.asa or trigger a session, which is a rather fundamental problem.