This article presents how to install Centos 5.
I assume you have already downloaded Centos Linux 5 64 bit(about 3.5 G) and you know how to use VirtualBox 64 bit(100M) which is available on OTN. Create virtual machine with default settings for Centos Linux 64 bit. 1GB ram and 32G for disk is enough to run Centos 5 64-bit. Rest of options you can keep default – however I usually increase memory for graphic card to 128M, enable 3D acceleration and set processor to 2.