I have exactly the same problem and have been using an old xp desktop machine as a workaround as well, but would like to have my W7 laptop going so I can take into shed. Did you find a permanent solution to this?
I have installed the driver Download | USBtinyISP | Adafruit Learning System but is not digitally signed but seems to install ok apart from the same.
Programmer Type : USBtiny
Description : USBtiny simple USB programmer, USBtinyISP - Inexpensive USB AVR Programmer
avrdude: programmer operation not supported