A Realtek rtl8139
is not a MODEM (the forum where you posted this thread), it's Network Adapter
chipset. Please read the many, many previously identical threads as it's already been asked and answered several times in this forum. A forum is for READING first and POSTING when nothing has been written about the subject previously.
Here's one of the last ones posted: http://forums.driverguide.com/showthread.php?t=25946