mgb I have tried a few usb to serila adaptors when programming plc's and some just don't work properly. It is better to spend on a real pci serial than a usb to serial convertor.
This has been done on laptops so we didn't have much choice but it is easy with a desktop.
That said I have not tried flashing the db with a usb to serial adaptor so don't know if they are as fussy with baud rates etc as the plc's
Sully