Turns out the maker had a buggy USB to serial driver. Since you didn't supply details I can't see if there is an updated driver. But do ask them.
I am trying to open a virtual serial (usb to serial) port in Hyperterminal and it tells me that it cannot open the COM port. I have tried the exact steps on other computers running Windows 200, XP, and Vista and it works just fine. This means it is not the device and has to be the individual computer, The drivers are loaded correctly, it enumerates and can be seen in device manager on the problem computer. My guess is there is another program blocking me from using it although there are other possibilities. I have tried multiple combinations of rebooting, reinstalling drivers, and killing background processes that may be accessing the virtual serial port.