flashing your device is really a simple task
b4 starting the flashing process you'll need to get 2 important files
1) the fw - the FirmWare for you device can be downloaded from here - http://tablets-dev.nokia.com/nokia_N900.php
just enter your imei number which is a 15 digits number located under your battery
2)the FW flasher - http://tablets-dev.nokia.com/maemo-d...-downloads.php
download and install it to your pc

now comes the fun part Flashing
1)move the FW file we downloaded earlier to the maemo flasher folder - by default it is located at c:\program files\maemo\flasher-3.5\
2)turn off your device
3)press and hold the U button on the N900 keyboard
4)while pressing plug the usb cable to the device and PC
5)run the flasher program - it is a cmd based software so you won't see any GUI
input the next command -
flasher-3.5.exe -F XXXXX -f -R
the XXXXX marks the place for the name of the FW file so just replace the XXXX with the File name of the FW ( usually it looks like this flasher-3.5.exe -F RX-51_2009SE_2.2009.51-1_PR_COMBINED_MR0_ARM.bin -f -R )
6)let it be - DO NOT touch the cable while it is working
7)after about ~4 mins your device is ready
8) let it reboot
You're Done