I haven't reflashed using Windows, but are you sure your holding the u key on the keyboard when booting up the N900?

The guide roughly should be the same. Make sure N900 has a near full charge, connect a turned OFF N900 to the computer (it might boot on, that's fine). Turn on the flasher, it should say no suitable USB device detected or something like that. Then unplug the N900, boot it up while holding the u button till you see the Nokia image with a USB icon in the upper right hand corner.

Then connect it to the PC and proceed with flashing.