You are pretty close @ica I think.

Was your device totally powered off before you inserted the USB cable?

The order is important here;
1. have N900 off and USB not connected
2. give command in PC prompt
3. connect N900 to USB

If this does not work then I think the problem is in windows, I cannot help with that because I have not had windows since XP long time ago.

Now as I think about it, does one have to do some sudo-like gimmic in windows too, like in linux? Does flasher require privilige to work?