I suggest you try with a linux image. Ubuntu Live CD should work too if you don't want to install. Download the 32-bit version. Aftyer that get flasher for linux

After installing it first run the message in the terminal without the -R part before connecting the device. Safer this way (ie: sudo flasher-3.5 -F <firmware-image> -f).

After running the command connect your N900 while pressing the u key. Keep pressing the U key for at least a minute. If the PC still doesn't start flashing it, release the U key. Try with different USB ports and if possible not a hub.