USB and RS232 ("Serial") devices run under similar specifications, although the connectors and cables used are totally different. If you want to connect a USB device to a serial port, this would have to be done using special equipment, and no other choice is really left to you, unless you want to go ahead and buy a USB peripheral attachment for your computer's motherboard. If you have no space in your motherboard left for such an attachment, you really cannot do anything other than get yourself a USB-to-RS232 converter--also known as a USB-to-Serial converter.
Turn on your computer normally and let it boot up on its own. Do not do anything until your computer is fully booted.
Attach the serial end of the converter to your computer's port and wait for your computer to detect it and install the necessary drivers. If your converter is simply a cable or some sort of simple device with just a USB receiver on one end and a serial connector on the other, then you might not have to wait for detection.
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Plug in a USB cable to the USB end of your converter.
On the other end of your USB cable, plug the USB device in which you intended to use on your computer. Wait a while and see if the computer detects it.