I couldn't. I have W7 Ultimate SP1 x64.
I tried several times, with different browsers, changing ports, etc.
It'd write a partition or two, reboot the phone and it would no longer be "visible" to the computer, so break the process.
In the end I decided to get my hands dirty and do the process manually, typing the commands in CMD, the old fashion way.
The hard side is you have to download the image, edit the
flash-all.bat file and clean it of everything other than the necessary commands. Then set the phone in fastboot mode and copy-paste the commands to CMD in same order.
It's an option.