Are these devices powered from the USB cable itself or do they have external PSU's? USB has an inbuilt design threshold per port current limit which prevents too much power being drawn by external devices, and it may be that these devices when used cause an overload (not serious in scary high voltage terms, merely outside the spec) which causes the psu to reset.
Some motherboards have a built in current limiter which turns off just the USB device at fault and raises an event log message or screen dialog (win xp, 2000 etc) to that effect. Try running them off an external psu or externally powered USB hub.
Do you have access to other pc's and if so can u try connecting these suspect devices to one of the other pcs. Preferably a different branded pc with a diff motherboard design.
keith
January 2006
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