If you're flashing on vista.
You may need to shorten the filename of the firmware. that was my problem when I start using flash on my n900 . my vista can't read long filename.
I rename mine OSPR1.2.bin

Put the FW file together with the flasher.exe

1.Turn off N900
2. Press "u" key on the keyboard and Plug the USB.
3.Once you see the unlit Nokia logo and the icon on topright you can now stop pressing the "u" key.
4. type cmd on the search box on windows. right click and run as admin.
5. run cmd.
6. type: cd "%ProgramFiles%\maemo\flasher-3.5"
7. after changing directory type:
flasher-3.5.exe -F OSPR1.2.bin -f -R (if you shorten the filename to OSPR1.2.bin)
If not type:
flasher-3.5.exe -F RX-51_2009SE_2.2009.51-1_PR_COMBINED_MR0_ARM.bin -f -R

you should be able to flash.