okay step by step
start from this part in the link i provided above
"Windows
N900
Updating the firmware using the Nokia Software Updater
Microsoft Windows users who wish to update their N900 can use the Nokia Software Updater (aka NSU), or use the Flasher (see section below).

The Nokia Software ............................."

what you will use is the maemo flasher
the link is provided there

and follow the steps after which goes as command nd the way to shut down your fone and then switching on the phone while holding the U key
its really simple honestly

a note: what windows are you using ? is it 32bit or 64bit ??
if you dont know how to get that info then just simply right click on the my computer icon and click properties
the info will be provided
the reason i ask is because 64bit systems have problem reflashing the phone

and if you need furthur help then im still around and if i reply late try searching Youtube