VirtualBox 4.2.6 on Fedora 18/17, CentOS/Red Hat (RHEL) 6.3/5.8

1. Change to root User
   su -
   ## OR ##
   sudo -i

2. Install Fedora or RHEL Repo Files
   cd /etc/yum.repos.d/
 
   ## Fedora 18/17/16/15/14/13/12 users
   wget http://download.virtualbox.org/virtualbox/rpm/fedora/virtualbox.repo
 
   ## CentOS 6.3/6.2/6.1/6/5.8 and Red Hat (RHEL) 6.3/6.2/6.1/6/5.8 users
   wget http://download.virtualbox.org/virtualbox/rpm/rhel/virtualbox.repo

3. Update latest packages and check your kernel version
   Update packages

   yum update

   Check that that you are running latest installed kernel version
   Output of following commands version numbers should match:

   rpm -qa kernel |sort |tail -n 1
 
   uname -r

   Note: If you got kernel update or run older kernel than newest installed then reboot:

   reboot

4. Install following dependency packages
   CentOS 6/5 and Red Hat (RHEL) 6/5 needs EPEL repository, install it with following command:

   ## CentOS 6 and RHEL 6 ##
   rpm -Uvh http://dl.fedoraproject.org/pub/epel/6/i386/epel-release-6-8.noarch.rpm
 
   ## CentOS 5 and RHEL 5 ##
   rpm -Uvh http://dl.fedoraproject.org/pub/epel/5/i386/epel-release-5-4.noarch.rpm
   yum install binutils gcc make patch libgomp glibc-headers glibc-devel kernel-headers kernel-devel dkms
 
   ## PAE kernel users install ##
   yum install binutils gcc make patch libgomp glibc-headers glibc-devel kernel-headers kernel-PAE-devel dkms

5. Install VirtualBox Latest Version 4.2 (currently 4.2.6)
   yum install VirtualBox-4.2

   Note:
   This command create automatically vboxusers group and VirtualBox user must be member of that group.
   This command also build needed kernel modules.

   Rebuild kernel modules with following command:

   /etc/init.d/vboxdrv setup
   ## OR ##
   service vboxdrv setup

6. Add VirtualBox User(s) to vboxusers Group
   Replace user_name with your own user name or some another real user name.

   usermod -a -G vboxusers user_name

7. Start VirtualBox
   Use launcher from menu or simply run:

   VirtualBox

Troubleshooting
   If you have problems with KERN_DIR parameter or your kernel directory is not automatically detected then set KERN_DIR environment variable manually, using following method:

   ## Current running kernel on Fedora ##
   KERN_DIR=/usr/src/kernels/`uname -r`
 
   ## Current running kernel on CentOS and Red Hat (RHEL) ##
   KERN_DIR=/usr/src/kernels/`uname -r`-`uname -m`
 
   ## Fedora example ##
   KERN_DIR=/usr/src/kernels/2.6.33.5-124.fc13.i686
 
   ## CentOS and Red Hat (RHEL) example ##
   KERN_DIR=/usr/src/kernels/2.6.18-194.11.1.el5-x86_64
 
   ## Export KERN_DIR ##
   export KERN_DIR