Quite an old node, but anyway...

You really should quit using any way of browser simulation to get the desired data: Siebel High Interactive Client does uses ActiveX and Java applets so it could behave like a full GUI.

Your best option is to use well defined interfaces like COM+, SOAP or the old method to post HTTP data and get an response. Of course, for the two latest options you would need to develop the interfaces from Siebel side. COM+ should be easier to implement, but it has it's drawbacks.