I'm not familiar with flashing it under windows environment but I'd check to see:
That you do not have virtual machines software installed and running. If you do not know what it is then you won't need to worry about it.
That you have the latest PC Suite software installed and all the approprite drivers.
That you are not messing around with the cable nor are there any other software that may get in the way with meddling the USB devices.
That you have the device in proper flashing mode. When the device is in flashing mode, you should see the USB logo on the top right part of the nokia logo screen static.If the USB logo momentarily appears and then disappears, you have not held the 'U' key down long enough or you pressed and held the 'U' key too slow.
3 x Nokia N900 | 2 x 32GB TopRAM microSDHC class 10 inserted in two devices | 2 x SIM cards inserted in two devices.
N900/ Nokia N900 is not a phone/smartphone! its an internet tablet with phone functionality - so quit assuming its still a phone/smartphone when it isn't!!
I use win7 32bit.
Help me....