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I'm trying to install Windows 2008 R2 (64 bit) on the Virtual Box 4.3.6. The extensions have been installed. The hosting machine is Lenovo t431u laptop with Wndows 8.1 Pro (64 bit). The i7 processor supports virtualization and it is enabled in the BIOS.
Once I start the virtual machine, two things happen:
First I receive a message that my display has 24 bit machine while vbox is optimized to use 32 bit settings (that's not true, my display has 32 bit color mode).
Second after the virtual box reads the Windows Disk, it gives me the following message:

Windows failed to start. A recent hardware or software change might be the cause.
To fix the problem:

1. Insert your Windows installation disc and restart you computer.
2. Choose your language settings, and then click "Next."
3. Click "Repair your computer."

If you do not have this disc, contact your system administrator or computer manufacturer for assistance.

File: \windows\system32\boot\winload.exe

Status: 0x000035a

Info: Attempting to load a 64-bit application, however this CPU is not compatible with 64-bit mode.

I attempted to restart multiple times. The Windows media is on the hard drive.

Please help.

I succeeded overcoming the problem. It seems that if you build the vm and change you host's BIOS settings, the vm does not recognize the change. I enabled cpu virtualization in the BIOS (Security section) and then re-built the virtual machine.

Nope. That wasn't it. What made it work was choosing a 64bit template when you rebuilt the 64bit VM.

The original VM was no doubt built with a 32bit template. After changing the BIOS setting you should have opened settings | General | Basic and changed the Version field from "Windows 2008" to "Windows 2008 (64 bit)".