I think I have found solution to the problem described above in case of arduino uno with CH340 USB-to-serial chip in Winodws 7 32 bit. Just update the COM driver with drivers for CH340 USB-to-serial chip which can be downloaded from DCcduino usb drivers (CH340 / CH341 chipset). Although the site describes a solution for Macbook, it contains also drivers for windows. This solution worked for me. (Exchange of usb CABLE and Arduino driver reinstalation did not work for me).
with regards
Bartek