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On the System/Motherboard tab you want to make sure you have enough RAM—2GBs for 32-bit at least 3GBs for 64-bits and that Enable IO APIC is on.
On the System/Processor tab, make sure you enable PAE/NX. And, if, and only if, you have a system with multiple core or processors, you can add another CPU to Windows 8. In my case with a six-core Intel Core i7, I gave Windows 7 two CPUs.
Under the System/Acceleration tab make certain that both Enable VT-X and AMD-V and Enable Nested Paging are checked.