okey just follow me here ,
pressing U key and look for usb sign at right corner.
check as if the driver for your windows successfully installed or not for this process( flashing parent driver,Usb Rom etc ,if says NOT SUCCESFULLY installed,you better install them on your own )
better rename this file :RX-51_2009SE_10.2010.13-2.VANILLA_PR_EMMC_MR0_ARM.bin ( for example : park.bin
now run the flasher with the command : flasher-3.5 -F park.bin -f -R
let the flasher ask for USB (as it will not be connected at that time )
when it asks ,simply connect the USB to N900 ,it should be working perfectly for ya .
last night i spent 4 hours bickering with this bloody driver issue .
Follow every thing and you will be okey .(every one else may follow these instructions .its nice and easy ) i hope so ;p