How can I download old versions of CentOS such as CentOS 6.3?

Download the .iso files here:

http://vault.centos.org

Getting Networking to Work with VirtualBox and CentOS

Why isn't my network working when I've installed CentOS 6 on VirtualBox?  Try to do this:

1) Edit the /etc/sysconfig/network-scripts/ifcg-eth0 file

2) Change

ONBOOT=no

to

ONBOOT=yes

3) Reboot the VM

Networking should work after that.