Flasher have some problem with windows which have x64 bits,
If you have 32 bits then follow the guide:

First switch off your N900
then, Hold U button and connect it to your usb (Your computer should recognize that your mobile is connected.)

Now, Open the maemo flasher-3.5

then write,
flasher-3.5.exe -F RX-51_2009SE_20.2010.36-2.004_PR_COMBINED_004_ARM.bin -f -R
And if you want to format your emmc then type

flasher_3.5.exe -F RX-51_2009SE_10.2010.13-
2.VANILLA_PR_EMMC_MR0_ARM.bin -f

Now its done,
Hope this help