You must have a device attatched that has some kind of "AutoOff" feature; just disable it. (digital camera, printer, etc. maybe).
If that's not the case:
Control Panel > Sounds and Audio Devices > Sounds > Device Connect/Disconnect > then choose "(None)" for the sound
So my computer keeps making the sound like a USB device has been connected, then the little symbol appears in the bottom right corner of the screen. Shortly after it will make the sound as though the device has been disconnected and the little symbol will disappear. The frequency of this varies, but sometimes it becomes very frequent and is rather annoying. I have a rather new Dell Inspiron 1525, and haven't had any previous problems. Any help would be appreciated. Thanks.