[Q] Problem with unlocking bootloader topic

I have a problem with Unlocing Bootloader of Sony Xperia P ( LT22i ) it doesn't go in fastboot mode when i press Vol + and plug the USB cable green light notification appears for 4 sec. and the mobile returns to charging mode and happens when i press Vol - too ..,, i use Fujitsu Laptop , Windows 7 ( 32-bit )

i want A help plz :) thnx ......