Day by day…

Well, sometimes things just stop working.

VirtualBox – error about modules not running + require to run as root:

 /etc/init.d/vboxdrv setup 

Well, Google will at 99% of places  recommend You to 

# yum install gcc kernel-devel kernel-headers

And it is help... But if not? 
Then it recommended to define KERNEL_DIR=
Well. sometimes it helps too, but what if not?

Well, let's cut here...
Here what I found as additional problem on RHEL & CentOS (& Fedora)
Nothing help? 

$ uname -a

Do Your kernel have .PAE ?
If Yes - then under root:

# yum install kernel-PAE-devel

and then

# /etc/init.d/vboxdrv setup 

Done.