After the NSU failed (stops after 33MB) I tried the Flasher 3.5.
Everything is clear, how to do it. http://wiki.maemo.org/Updating_the_firmware

Starting the flasher in the shell:
"flasher-3.5.exe -F RX-51_2009SE_10.2010.12-9_PR_COMBINED_MR0_ARM.bin -f -R"
brings a few line with %m. No further comment. No action at all, the program simply stops.
The same result is when the N900 USB is not even connected.

IMHO the problems starts when connecting the N900 (pressing "u"), Windows says it has found a new device but at the end, an error comes (hardware problems with USB device).
I tried also a laptop with XP, the same here. For me the flasher doesn't install properly the USB driver.

I have Windows XP. Also tried Vista bit still without any SP (will do the update as wiki says you must have SP2).

Conclusion: Flasher 3.5 is bugy !!
No way to update to process the update. Incredible !!
I cannot imaging that I'm the only one with that problem.

Sorry there is another thread about this but I couldn't find it again.