Just treat it as a standard serial port: Open it, set up a receive data event handler, and process the data.
The only complication you might get is if the RFID reader does any data blocking, and / or encryption, but I can't help you there as I have no idea about the specific reader you are using.
private SerialPort RFID;
RFID = new SerialPort();
RFID.PortName = "COM1";
RFID.BaudRate = 9600;
RFID.DataBits = 8;
RFID.Parity = Parity.None;
RFID.StopBits = StopBits.One;
RFID.Open();
RFID.DataReceived += new SerialDataReceivedEventHandler(RFID_DataReceived);
private void RFID_DataReceived(object sender, SerialDataReceivedEventArgs e)
{
string data = RFID.ReadExisting();
myTextBox.Text += data;
}
Should get you the raw data; it's then up to you to process it appropriately.
[edit]I can't even spell "object" this morning: corrected - OriginalGriff[/edit]