Another way to update firmware is to do it in command line:
Step 1. Install WinAVR: http://sourceforge.net/projects/winavr/files/WinAVR/20100110/
Step 2. Open a command line window, cd to where your osFWUpdater is, and cd further to Firmwares\OpenSprinkler_v2.0
Step 3. Run command
avrdude -c usbtiny -p m644p -F -B 1 -U flash:w:firmware2.1.2.hex
Of course the pre-assumption is that the device has to show up as USBtiny (or USBtinySPI) in Windows — if it shows up as unknown, it won’t work.