USB devices stop after idle

USB devices stop after idle
We have a Compaq TC-1000 tablet PC with WinXP tablet addition installed on it.
When the user stops working for a while, just a few minutes, the USB devices sometimes stop working. Since it is the externeral mouse orKeyboard it’s a problem. However if we unplug them then plug them in again alls well til the next time. Note on tablet PCs there are no PS/2 ports or other legacy connectors. The specific hardware is a MS natural keyboard and a Belkin mini optical mouse. Any ides?