How to get N900 flasher to work in Windows 7 64-bit (x64).
Install Flasher 3.5 - http://skeiron.org/tablets-dev/
Download libusb-win32-bin-1.2.2.0.zip - http://sourceforge.net/projects/libusb-win32/
Press U while starting N900 and attach USB
Wait for the original driver to install.
Run inf-wizard.exe (bin folder in zip) - Run as Administrator
Press "Next"
Search in list and select "0x0421 - 0x0105 - Nokia N900 (Update Mode)"
Press "Next"
Press "Next"
Save .inf file
Press "Install Now"
Press "Install this driver software anyway" on warning message.
You should get "Installation successful." and press OK.
Check Windows Device Manager for "libusb-win32 devices", expand and verify "Nokia N900 (Update Mode)" can be found.
Now you are ready to run Flasher 3.5 with your parameters.

then

http://wiki.maemo.org/Updating_the_firmware

bonus:

http://lmgtfy.com/?q=how+to+change+directory+cmd

EDIT: flasher link updated