Here the answer!!!!!
reset your BH503 headset...(press multifunction key hold and press volume up key until the three colour red,gree,blue blinking)...go to bluetooth icon tray,right click and add device...when the BH503 appear PLEASE DON'T Click NEXT first..but right click at the bluetooth headset icon...tick "headset/handsfree/stereo" in the box...and then you can click NeXT... while it's installing quickly PLEASE CLICK 'SKIP OBTAINING UPDATE FROM MICROSOFT' in blue link or it will update unwanted #$%^^... just wait...when its finished, go to sound icon,right click go to sound..click Playback Tab, find the suitable Bluetooth Headset and right click ... click Set As Default...and tadaaa!!!! its works for me!!!!!!!