My HP 2710p could not start Windows 7 64 in VirtualBox,
VT-x/AMD-V hardware acceleration has been enabled, but is not operational. Your 64-bit guest will fail to detect a 64-bit CPU and will not be able to boot.Please ensure that you have enabled VT-x/AMD-V properly in the BIOS of your host computer.
This error message appeared, even when I enabled Virtualization Technology in BIOS.
I fixed it by updating BIOS version, it runs OK, no problem after BIOS update. Find instructions for BIOS update on official HP support site or instructions for BIOS update for non-windows users here.
BTW my VirtualBox System settings for the Window 7 64 machine on HP 2710p as a host: Motherboard: Enable IO APIC is checked and in Acceleration tab, both Enable VT-x/AMD-V and Enable Nested Paging is checked.