Speakers & Headphone setup

I have a Toshiba notebook running Vista Ultimate. When I first got the notebook, the built-in speakers were on by default (of course) however when I plugged a headset into the jack, the headset took over the sound and the built-in speakers turned off. When I unplugged the jack, the built-in speakers came back on. This was good.

Now, when I plug a headset into the jack, the headset speakers don't come on automatically. In an effort to troubleshoot, I have been monkeying around with the settings and drivers. The drivers are all up to date. My devices are enabled. I have been able to get sound through the headset and/or the built-in speakers independently. But I have not been able to configure my setup as described above - which is what I am after. I just can't seem to get the seetings right.

I will appreciate any help.

Thanks,
Tom