Getting VirtualBox Additions to install on CentOS release 5.4 (final):

 yum install gcc -y

 yum install kernel sources -y

 sh ./VBoxLinuxAdditions-x86.run

 Verifying archive integrity... All good.
 Uncompressing VirtualBox 3.1.4 Guest Additions for Linux........
 VirtualBox Guest Additions installer
 Removing installed version of VirtualBox Guest Additions...
 Building the VirtualBox Guest Additions kernel modules
 
 Your guest system does not seem to have sufficient OpenGL support to enable
 accelerated 3D effects (this requires Linux 2.6.27 or later in the guest
 system).  This Guest Additions feature will be disabled.
 
 Building the main Guest Additions module                   [  OK  ]
 Building the shared folder support module                  [  OK  ]
 Doing non-kernel setup of the Guest Additions              [  OK  ]
 Starting the VirtualBox Guest Additions                    [  OK  ]
 Installing the Window System drivers
 Installing X.Org 7.1 modules                               [  OK  ]
 
 You appear to be running an older version of the X Window system in your
 guest.  Seamless mode and dynamic resizing will not work!
 
 Setting up the Window System to use the Guest Additions    [  OK  ]
 You may need to restart the hal service and the Window System (or just restart
 the guest system) to enable the Guest Additions.

 Installing graphics libraries and desktop services componen[  OK  ]

 reboot

BUGS

Every time the environment changes resolution during the boot, (if you have multiple monitors) the VM will always go back to monitor zero.

See Also VirtualBox Shared Folders