How to hard reset Nokia N900

I Before You Begin

1. This will completely erase everything on your Nokia N900 and get it back to exactly the way it was when it arrived in the box.
2. This is perfect for people reselling their Nokia N900 or just want to start from scratch.
3. THIS DOES NOT WORK ON WINDOWS 7 BECAUSE OF USB ISSUES. FIND A WINDOWS VISTA OR LOWER SYSTEM TO USE TO DO THIS!

II. Download the Firmware, eMMC, and Firmware Flasher

1. Download the Flasher 3.5 program from here and then run the .exe (at the time of writing this, it is called “maemo_flasher-3.5_2.5.2.2.exe”:
Flasher 3.5
2. Then download the latest firmware for your phone here (it will be a .bin file and say Latest then your region next to it) AND download the eMMC image AS WELL (it will also be a .bin file but will look like this “RX-51_2009SE_1.2009.41-1.VANILLA_PR_EMMC_MR0_ARM.bin”)
N900 Firmware and eMMC (it will ask you for your phone’s IMEI to continue)
3. Save both the .bin files into the Flasher 3.5 folder that was created by the flasher-3.5.exe program you installer earlier (which should be Program Files > Maemo > Flasher-3.5).
4. Rename the two files like so (to make life easier later):
Latest firmware – rename it to “firmware” (no quotes)
eMMC file – rename it to “emmc” (no quotes

III. Flash the Phone

1. Turn off the phone
2. Hold down the U button on the keyboard of your phone and while holding it plug in the USB cable connected to your computer. (You should see a very DIM Nokia screen with a little USB sybol in the top right corner on the phone).

3. Open the command prompt on your computer by going to start and typing in cmd into the search box and hitting enter.

4. First we flash the firmware file, type in the following commands hitting enter at the end of each line:

cd\
cd c:\programfiles\maemo\flasher-3.5\

(*if you are using a 64 bit system, replace “program files” with “program files (x86)”)

flasher-3.5.exe -F firmware.bin -f -R 

(*if you get a %m error after typing this, go back to the files and .bin after their name and try again. Also try not having the -R at the end.)

5. Leave the command prompt open and wait for the phone to finish flashing and rebooting, then turn off the phone.

6. Hold down the U button on the keyboard of your phone and while holding it plug in the USB cable connected to your computer. (You should see a very DIM Nokia screen with a little USB sybol in the top right corner on the phone).

7. With the command prompt still open on your computer from the firmware flash, we can now flash the eMMC file. Type in the following commands into the command prompt with hitting enter at the end of each line:

flasher-3.5.exe -F emmc.bin -f -R  

(*if you get a %m error after typing this, go back to the files and .bin after their name and try again. Also try not having the -R at the end.)

9. Wait for the phone to reboot. All done! Phone is now back to out of box state.

credit to  http://theunlockr.com/2010/04/02/how-to-hard-reset-your-nokia-n900-get-it-back-to-out-of-box-state/