Install Flasher 3.5
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. every time u fail u must load lib-usb again !

IF THIS FAILS ! : figure out somehow ( pull patery, insert whit preset button and cable) PC or lib-usb must detect n900 so you need download few drivers and if u will fail ENABLE RD mode in your flasher by comand : ./flasher.exe --enable-rd-mode and do it all again
u wellcome !