New
#11
This is the first time you have said what is actually going on! Now we can go somewhere with your problem! I have never seen USB devices that "go on & off", and I assume you are talking about the System Tray icon for the alert messages.
Normally when you plug-in a USB device, the alert appears, the driver is installed, the PC is happy. Since you apparently are not plugging anything in when the disruption occurs, then I have to suspect the USB devices you do have, namely the mouse and keyboard, that one or both of these is flaky and/or has a bad cable (and it's unplugging/replugging itself by virtue of a bad connection). If you've tried a new kb and mouse and then uninstalled the "unknown devices" again and still have the problem, then I guess I'd start suspecting the motherboard.
What mfr/model of keyboard & mouse are you using?