Why do I get a "there are no active mixer devices available" error? I lost audio and can't get it back.

I have no sound capability. There is a "no active device" in the audio "volume" properties header and under "sound" properties the default playback and recording devices drop down windows are all grayed out .

No fix by uninstalling and reinstalling the Realtek AC'97 audio device or by downloading the latest driver from Intel the motherboard manufacturer. I also checked that the onboard audio device is enabled in the BIOS. Odd thing is that the system sees everything ok and in device manager all audio devices are showing "this device is working properly".

This began after I hooked up a Xitel INPort USB audio device to record from LPs. Had no problem initially but it started after re-starting the computer with the device hooked up. The problem persists even after disconnecting the USB device.

Has anyone had a similar experience? I have an HP Pavilion desktop with XP home edition. I don't know what else I should try. Thanks.