trying to reconnect to my Kindle via USB

My Kindle Fire HD 7 software was recently updated to Fire 05.5.1.1. I now seem unable to use the USB cable to connect my Kindle unit to the Calibre 2.52.0 version on my main desktop computer running Windows 7 Home Premium Service Pack 1.

Yet when I connect the same unit to my Portable computer running Vista Home Premium Service Pack 2, the USB interface works fine.

I used to be able to simply connect my Kindle unit to my desktop computer via USB cable ; Calibre would automatically acknowledge the unit and the titles previously downloaded to the Kindle would appeared as "On Device" in the Calibre interface.

All my e-books before that were sent via USB cable from my desktop Calibre software to my Kindle unit running CC; then, all of a sudden, the only connection I could activate is the wireless.

Even that is unstable : if the unit is left unattended for a while, the "On Device" column disappear from the Calibre interface. I must then manually disconnect then reactivate the wireless option.

I have disable my anti-virus to check if the problem was there but it did not change anything.

Any suggestion on how to get the USB connection back on track ?