This virtual machine is configured for 64-bit guest operating systems. However, 64-bit operation is not possible

Solution 1:

Do what the man said: Enable VT-X in the Bios.
Sometimes it is also called Hyper-V. (For some obscure reason it often is hidden in the "security" section of the Bios.)

Most computers ship with the setting disabled by default. Some computers (especially laptops) are technically capable of VT-x but the Bios doesn't show it so you can't enable it.
In that case you are sheer out of luck.

Solution 2:

I get this error every time my Windows 7 Dell desktop restarts after a windows update. My CPU does support vitualization, and the BIOS is properly configured, but somehow the restart puts it in a weird mode.

Simple shut-down, and power on fixes the problem.

Solution 3:

Go to BIOS by pressing ESC on powering on. Go to System Configuration -> Device Configuration or similar Check the enable VT-x Save Power cycle Should work