Cannot Enter to flash mode nokia n900
 

pls help me i want to flash my nokia n900 but when i try to put it into flash mode by pressing U key (switched the device off before) and connectin the usb cable it starts installing drivers and showing that usb icon on display of the phone but after few seconds it disappears and windows plays the sound of disconnecting drive and it connects as normal drive and the device is just charging battery after the startup dots :S :S :S

Windows xp 32bit My Pc's OS
Nokia n900 cssu and overclocked 1000MHz