This is the exact filename of the FW update you need...
RX-51_2009SE_10.2010.19-1_PR_COMBINED_MR0_ARM.bin

If you flash vanilla you will indeed lose all data but if you only flash the fw then you will not lose data.
Please let me know if you need further instructions on flashing.

And do a backup with the utility on your device first before flashing and once you have flashed successfully do a restore if you want to from the backup you made.

Here is the guide that mackan1 put together for anyone that needs to flash and remember you only usually need to flash the FW... NOT the emmc (vanilla).

My step by step on how to Flash the N900 on windows XP SP3+
This will format the whole phone so everything will be lost unless you back it up.
Formats eMMC images and flashes the phone to 1.2 Global Release.

1. Downloaded (http://tablets-dev.nokia.com/nokia_N900.php)
• flasher-3.5.exe
• RX-51_2009SE_10.2010.19-1_PR_COMBINED_MR0_ARM.bin
(http://tablets-dev.nokia.com/nokia_N...ED_MR0_ARM.bin)
• RX-51_2009SE_10.2010.13-2.VANILLA_PR_EMMC_MR0_ARM.bin
(http://tablets-dev.nokia.com/nokia_N...MC_MR0_ARM.bin)

2. I installed the "flasher-3.5.exe"
3. I then moved
"RX-51_2009SE_10.2010.19-1_PR_COMBINED_MR0_ARM.bin"
"RX-51_2009SE_10.2010.13-2.VANILLA_PR_EMMC_MR0_ARM.bin"
To the directory where I installed the flasher (Default is C:\Program Files\maemo\flasher-3.5).
4. I then done a full backup of my N900 using the backup tool on the phone and moved all my stuff on to the memory card.
5. Ensure the battery is at least half full.
6. Unplug charger cable and switch off the device.
7. Open the Command Prompt (Start then Run or Windows Logo key + R) and type "cmd" then press Enter (this will open a black window).
8. Change the directory to the Flasher's path by typing in "cd "%ProgramFiles%\maemo\flasher-3.5"
9. Hold the u key on the device keyboard and connect the USB cable at the same time.
10. Run the following command by typing:
"flasher-3.5.exe -F RX-51_2009SE_10.2010.19-1_PR_COMBINED_MR0_ARM.bin -f"

After this completes

11. Run the following command by typing:
"flasher-3.5.exe -F RX-51_2009SE_10.2010.13-2.VANILLA_PR_EMMC_MR0_ARM.bin -f -R"

The flash will take around 1 minute after which the device will reboot.
The bright Nokia screen will come up on the screen with the USB icon on the top right followed by progression dots with yellow LED blinking for a few seconds
And you’re done. Welcome to the world on 1.2

Here is my step by step.
This was the 1st time i flashed my phone. only took me about 5mins in total after i had downloaded all the things i needed.

and using copy and past (in CMD bit u can have right click to copy)

Hope this helps

Please take note that an OTA (over the air) update does not always work simply because of the verify situation and can leave your N900 in a reboot loop or a refusal to start up DON'T PANIC you cannot do any damage whatsoever to your device either by OTA or flashing manually and something goes wrong.
The way forward from these kind of situations is to just take your time and do a manual flash again if need be untill the N900 accepts the flash and starts up normally.
If ever you get slow reactions from the N900 after it reboots itself from being flashed, simply wait till it has done the cycle of rebooting and turn off and back on again.
To get back your apps after a successful flash simply do a restore from the backup you did prior to flashing follow the prompts and it will re-install all your apps automatically.
It is actually IMPOSSIBLE to "brick" the N900 from flashing in any way or form please remember this.
All faulty for whatever reason flashing can be done again with no ill effect whatsoever to your N900 and as many times as you want or need to flash.

first, thanks for helping me and all the info you supply.
second, thanks for relaxing me... well, i guess you know what is the feeling when your phone stops functioning and you don't know what's going to be with your data.

i was able to do a backup before installing that broken update.
i've also copied right now all the info appear in my Nokia N900 when i see it on my computer in mass storage mode.

can i do more (regarding backup etc)?

now for the flashing stuff - i have the following status:
I entered the command in the cmd like the following and nothing happened. This is what I got:

D:\Program Files\maemo\flasher-3.5>flasher-3.5.exe -F RX-51_2009SE_10.2010.19-1_PR_COMBINED_MR0_ARM.bin -f -R
flasher v2.5.2 (Sep 24 2009)

SW version in image: RX-51_2009SE_10.2010.19-1_PR_MR0
Image 'kernel', size 1705 kB
Version 2.6.28-20101501+0m5
Image 'rootfs', size 173568 kB
Version RX-51_2009SE_10.2010.19-1_PR_MR0
FIASCO subimage data length mismatch

and the N900 is in flash mode (can see the usb icon on the upper right corner).

Your flashing in Ubuntu yes?

i'm flashing from a Windows XP machine and the Nokia N900 have Debian distro like on it.

should i use the flahser from different OS and not from Windows XP?
the flahser i download was for windows XP.
what is the relation/your question to Ubunto?
i can't handle the problem using windows XP?

there's a mismatch notification in the cmd command i've added.
what should i see in the output after running the command?
is this the right .bin file i should use?

thank you again!

You're a life saver abill_uk

I succesfully fixed my N900 on windows 7. I had the same BS problem..

should i use the flasher from C drive or it does not matter?
right now its installed on D drive.

Sorry for my delay in replying as something is going on with a mod on here and got me engrossed lol... ok back to topic..
Are you using 64bit xp?
When you run the flash command you should only see the countdown as it is doing the flash not the mismatch in the file your trying to flash.
What is the command you used to flash?
what is the actual file your using to flash? (filename please).