Open this link, download maemo_flasher-3.5_2.5.2.2.exe and install it.
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.
Download the last link, it should be the GLOBAL firmware 51.(Unless you want the middle eastern one or USA)
Save the file to C:\Program Files\maemo\flasher-3.5
Put the battery back in, and press the "u" key on the keyboard of your N900, don't leave it.
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.
Go to Start->All Programs->Maemo->Maemo Flasher 3.5->Maemo Flasher 3.5.
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.
here is what i did to charge my battery since the power adaptor was not doing anything, charged it for about an hour, put the battery back in hold "U" and now you will see the USB logo and then proceed flashing the phone use the Flasher
I don't think that is exactly the safest/most officiant way to charge a lithium polymer battery unless you know exactly how much power comes out of that USB port
(I've seen people ruin batteries from doing things similar to this )
I had the exact same problem, NSU said 1 sec remaining for about an hour, batter went very low and the phone wouldnt charge, and I couldn't boot into USB mode holding down the 'u' key.
The solution for me was to use a charged battery, and then use meamo flasher 3.5 as described.
A few point worth mentioning..
almost all nokia batterias use 3.7V, so if you can charge your N900 battery, see if you can find another 3.7V battery from another nokia phone that you can fit it, as long as the contacts are touching it should be all good. I used a BL-4C from an old 7370 and it worked a treat.
Also when using meamo flasher 3.5, make sure you observe all the spaces when typing the flash command, other wise it wont like it. not too sure if the command is case sensitive but copy it exactly to be on the safe side.
Mine is all back up and running now, to my relief. I'd like to say thanks to everyone on here for the helpful comments, much appreciated!
If my instructions are unclear please let me know and I'll try to clarify them for you
)