IIS 7.5 wont load web service javascript

Im having an issue getting my webservice javascript loaded via a script manager

I register a service using the following code:

PageScriptManager.Services.Add(new ServiceReference("~/Services/ClientScriptService.asmx"));

which in turn generates an html script block

<script src="Services/ClientScriptService.asmx/js" type="text/javascript"></script>

This has worked for ages but now I get a 404 error from Services/ClientScriptService.asmx/js when I load the site onto Windows Server 2008 R2 which is running iis 7.5 It works fine on my windows 7 box

any ideas how to fix this?

In iis manager, i changed the Managed Pipeline mode of the app pool to integrated and it all started working... but I dont understand why