[ubuntu] VM ware error

i'm unable to install Virtual box 64bit in my Pc.

Specifications
Ram: 4GB
processor : Core2Duo
Motherboard : ASUS P5G41T-MLX

my board supports 64Bit Os, i installed Ubuntu 12.04 64bit OS . The problem occurs when i try to install Virtual box , it automatically installs 32 bit virtual box instead of 64 bit Virtual box , even my system supports 64bit and i unable to install any 64 bit OS in my virtual Box. What is the problem?
some screen shots
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