Code snippet from here: http://support.microsoft.com/kb/823179

open_lpt.vb

' This sample from Microsoft does not work on Windows 7 or Server 2008
' Calls to OpenFile and WriteFile fail silently
' The API calls are supposedly supported in Win 7 and Win 8
' Please offer any suggestions in comments


 ' Obtain a handle to the LPT1 parallel port.
         hParallelPort = CreateFile("LPT1", GENERIC_READ Or GENERIC_WRITE, 0, IntPtr.Zero, _
            OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, IntPtr.Zero)
         ' Verify that the obtained handle is valid.
         If hParallelPort.ToInt32 = -1 Then
            Throw New CommException("Unable to obtain a handle to the LPT1 port")
         End If
         ' Retrieve the current control settings.
         Success = GetCommState(hParallelPort, MyDCB)
         If Success = False Then
            Throw New CommException("Unable to retrieve the current control settings")
         End If
         ' Modify the properties of the retrieved DCB structure as appropriate.
         ' WARNING: Make sure to modify the properties according to their supported values.
         MyDCB.BaudRate = 9600
         MyDCB.ByteSize = 8
         MyDCB.Parity = NOPARITY
         MyDCB.StopBits = ONESTOPBIT
         ' Reconfigure LPT1 based on the properties of the modified DCB structure.
         Success = SetCommState(hParallelPort, MyDCB)
         If Success = False Then
            Throw New CommException("Unable to reconfigure LPT1")
         End If
         ' Retrieve the current time-out settings.
         Success = GetCommTimeouts(hParallelPort, MyCommTimeouts)
         If Success = False Then
            Throw New CommException("Unable to retrieve current time-out settings")
         End If
         ' Modify the properties of the retrieved COMMTIMEOUTS structure as appropriate. 
         ' WARNING: Make sure to modify the properties according to their supported values.
         MyCommTimeouts.ReadIntervalTimeout = 0
         MyCommTimeouts.ReadTotalTimeoutConstant = 0
         MyCommTimeouts.ReadTotalTimeoutMultiplier = 0
         MyCommTimeouts.WriteTotalTimeoutConstant = 0
         MyCommTimeouts.WriteTotalTimeoutMultiplier = 0
         ' Reconfigure the time-out settings, based on the properties of the modified COMMTIMEOUTS structure.
         Success = SetCommTimeouts(hParallelPort, MyCommTimeouts)
         If Success = False Then
            Throw New CommException("Unable to reconfigure the time-out settings")
         End If
         ' Write data to LPT1.
        	' Note: You cannot read data from a parallel port by calling the ReadFile function.
         Console.WriteLine("Writing the following data to LPT1: Test")
         Success = WriteFile(hParallelPort, Buffer, Buffer.Length, BytesWritten, IntPtr.Zero)
         If Success = False Then
            Throw New CommException("Unable to write to LPT1")
         End If

I found the solution here: http://blogs.lessthandot.com/index.php/DesktopDev/MSTech/printing-to-a-zebra-printer