I was trying to setup VirtualBox on my Athlon 2600+ server under my desk to run Windows XP. The following are the steps which I used to install and use Virtual Box:

Install VirtualBox on CentOS 5

wget http://download.virtualbox.org/virtualbox/4.0.8/VirtualBox-4.0-4.0.8_71778_rhel5-1.i386.rpm
yum -y install libGLU SDL
rpm -ivh VirtualBox-4.0-4.0.8_71778_rhel5-1.i386.rpm

Setting up a VM

As per the documentation at: http://www.virtualbox.org/manual/ch07.html#idp11472496

VBoxManage createvm --name "WindowsXP" --ostype WindowsXP --register
VBoxManage modifyvm "WindowsXP" --memory 256 --acpi on --boot1 dvd --nic1 bridged --bridgeadapter1 eth0
VBoxManage createhd --filename "WindowsXP.vdi" --size 10000
VBoxManage storagectl "WindowsXP" --name "IDE Controller" --add ide --controller PIIX4
VBoxManage storageattach "WindowsXP" --storagectl "IDE Controller" --port 0 --device 0 --type hdd --medium "WindowsXP.vdi"
VBoxManage storageattach "WindowsXP" --storagectl "IDE Controller" --port 0 --device 1 --type dvddrive --medium /mnt/box/downloads/OS/windows_xp-sp3.iso
VBoxManage modifyvm "WindowsXP" --vrde on

To start the newly created virtual machine, run:

VBoxHeadless --startvm "WindowsXP" 

Making Remote Desktop Work

By default, the Virtual Box install does not come with the extension pack which enables VRDE. If you attempt to start the VM with VRDE enabled:

VBoxHeadless --startvm "WindowsXP" --vrde on

You will see this error in your log file:

VRDE: VirtualBox Remote Desktop Extension is not available.

To install the extension pack, download the latest extension pack at: http://download.virtualbox.org/virtualbox/

For example:

wget http://download.virtualbox.org/virtualbox/4.0.8/Oracle_VM_VirtualBox_Extension_Pack-4.0.8-71778.vbox-extpack
VBoxManage extpack install Oracle_VM_VirtualBox_Extension_Pack-4.0.8-71778.vbox-extpack

Changing the DVD drive ISO

First, unmount the image by running:

VBoxManage storageattach "WindowsXP" --storagectl "IDE Controller" --port 0 --device 1 --type dvddrive --medium emptydrive

then remount the drive as shown above.