Welcome to Maemo Community. please always do a quick search or power search first before asking any question. there are dozens of threads available for flashing N900.

For flashing please consider checking wiki HERE. and if it still didn't helped you then proceed with THIS step by step guide. If you are running windows XP then make sure that you are running Service Pack 3 as drivers doesn't install properly in Service Pack 2 or use Windows 7 32 bit.