Try cold flash. Here are "2nd" and "secondary" parts.

Remove battery, run flasher-3.5 in coldflash mode, connect n900 with usb cable.. and then you need to insert-remove-insert-remove-insert battery until operation will be done (you may notice some vibration and debug info in console). It's hard for first time. (no dick jokes)

P.S. there may be a lot of windows popups about different plugged devices. Also you need to remove-insert usb cable, if windows stops beeping when the battery is inserted.