Error occured while loading a new bin to ...

I am having difficulty with my computer serial port, while loading a new bin to my viewsat xtreme .Zquint appears then "Error occurred " msg appears. Null modem cable is original I follow all the instructions exactly as it is mentioned in the satellite forums. my computer configuration is celeron 633mhz with XP os. but if i use the same null modem cable with different computer its just loads fine.
i have checked in the device manager com port is working fine, i uninstall and loaded driver as well as new driver, checked with all the comport # , use.changed baud rate too. no use , whats wrong with my computer?
Any help is greatly appreciated