Microsoft has acknowledged an issue with USB 2.0 Bluetooth devices that stop working intermittently when connected to computers running Windows Vista Service Pack 2 (download). “If a Bluetooth dongle is connected to a USB 2.0 hub, the Bluetooth device may stop working intermittently. There’re limited USB ports available on some chipsets. Therefore, manufacturers sometimes build internal USB 2.0 hubs into their computers.” 

To fix: If your portable computer has a Bluetooth switch, use the Bluetooth switch to enable and disable the Bluetooth radio. Desktop users disconnect and then reconnect the Bluetooth dongle.  To disable and enable Bluetooth radio: Click Start, click Control Panel, and then click Device Manager, Right-click the Bluetooth radio, and then click Disable, After the Bluetooth radio is disabled, right-click the Bluetooth radio, and then click Enable, explained Microsoft in KB970408.