It happens that I need to downgrade my laptop with windows 8 64-bit and it gives me a headache.
Anyway for those users who wants to downgrade their PC or laptop win 8 64-bit enabled device you may do the following.
1. Backup your Disk, make bootable backup or iso.
2. Prepare your win 7 cd
3. Insert your cd and boot to win 7
4. Do the instruction on this link specially the part of diskpart
thanks Kent Chen
5. then continue installing win 7.
Have a great day ^___^