1. Unplug the device.
2. Open this link, download maemo_flasher-3.5_2.5.2.2.exe and install it.
3. Open this link, open your device and take the battery out, just like in the picture (in the link), type the IMEI number and accept the terms.
4. Download the last link, it should be the GLOBAL firmware 51.(Unless you want the middle eastern one or USA)
5. Save the file to C:\Program Files\maemo\flasher-3.5
6. Put the battery back in, and press the "u" key on the keyboard of your N900, don't leave it.
7. Connect the phone to the computer, keep holding "u" until you see the nokia logo with a small usb icon on the top right corner.
8. Go to Start->All Programs->Maemo->Maemo Flasher 3.5->Maemo Flasher 3.5.
9. Type this: "flasher-3.5.exe -F <FILENAME> -f -R", instead of <FILENAME> you should type the name of the file you copied to the flasher folder (something with .bin, including .bin).

Give it a few minutes, if you followed the instructions everything should be OK. Good luck.

More Info: http://wiki.maemo.org/Updating_the_f..._.2F_Windows_7