virtualbox.org

My real machine's cpu is intel core 2 duo Q8200? 4-Cores?64bit.
My host-os is fedora13-64bit.
My guest-os is win-2008-r2-64bit.
Can't-Work.
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My real machine's cpu is intel core 2 duo Q8200? 4-Cores?64bit.
My host-os is fedora13-bit.
My guest-os is win-xp-32bit.
Can-Work.

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Why?

Q8200 does not support VT-x. Running 64bit guest in VirtualBox requires VT-x.

So we can make a conclusion: your processor can not run 64bit guest in VirtualBox due to lack of VT-x.