Modem Baud rate

I have a Comaq Armada 100s, Laptop, AMD K6 500mhz, 64mb, 4gb hd, 56k internal modem, Win98.
I have tried connecting an external modem to one of the serial ports but the best baud I can get is 2400, (should be 11500). Is this conflicting with the other modem, if so how do I disconnect it?
Could there be something wrong with the port? (I have a serial port tester, (small plastic piece that fits onto the port) but have no idea how to use it!
Please help!!