I've a PARALLAX RFID sensor attached to my UNO? board, running on Windows 7, Arduino 1.0.5.
Attached how?
ReadSerial(RFIDTAG);
What is this function doing? The name implies that it is reading serial data, but Processing is on the other end of the serial port, so that doesn't make sense.
Serial.write(firstSensor);
Serial.write(secondSensor);
You are sending binary data.
You have not provided near enough code to actually help you.