What usb to serial adaptor? The only one I use and recommend is the iogear guc232a, others may work but that one has never failed me. Have you tried another URC cable? Sometimes they go bad too. And have you gone into the device hardware settings in XP to check when it is plugged in that it is recognized and operating correctly? Are you sure the usb driver is loaded properly for the adaptor? Tried to uninstall and reinstall the driver? Tried using the serial adaptor on another device to communicate with via terminal or other serial communication software?