FTDI's 3rd generation chip, the FT2232C, is a composite device with two ports that can be configured to behave as a USB UART or a USB FIFO interface by configuring an external EEPROM. In addition, the FT2232C EEPROM determines which drivers the device will use.
The EEPROM is read on power-up and will configure the chip accordingly before the device is enumerated. If no EEPROM is fitted, the device will default to both channels configured as USB to UART interfaces using VCP drivers.
Under Windows 98 and ME, the Found New Hardware Wizard should be used to install devices when they are connected to the PC for the first time as this is the recommended procedure for reliable device operation.