Go Start->Control Panel->Hardware and Sound->Sound and on the Playback tab select the proper speaker set, then click the "Set Default" button. You may have to repeat with the Recording tab.
Hope this helps,
John
I recently plugged in a friends speakers into my laptop for music reasons but I gave them back and now my computer says that no audio output device is installed. I figured it reset its default to the speakers and I need to change it back, but be that the problem or not, how do I fix it? I have windows vista on an HP Pavilion Enterainment PC (HP Pavilion dv2000). Thanks all.

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