This is a general problem with the low-cost Prolific processor chip and drivers. You should invest in a better USB to Serial adapter (http://www.usconverters.com/index.php?main_page=product_info&cPath=67&products_id=244) with a FTDI chipset. It costs a bit more but its well worth the money. The drivers for the FTDI chipset are well documented and compatible with most devices.
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