This may not be your problem but this is a great place to make an important comment.
I do a LOT of serious development work in my day job and have to use USB-RS232 cable every day for many things.
Many cable do not work correctly, including ones based on the Prolific chipset.
As far as I can determine, the "gold" standard is the cables based on the FTDI 232 chipset. Here is a cable based on this chipset.
http://www.usbgear.com/computer_cabl...1,106,1009,601
Another one is:
http://www.rabbitsemiconductor.com/p...le/index.shtml
My experience has shown that the IFI downloader works reliably ONLY with the cable based on this chipset. Same scenario with the Rabbit and a few other toolsets.
Ed