Backtrack 5 is dedicated operating system for penetration testing. This is perfect for forensic. Download ISO from here.
Backtrack is must have for information security field. It contains bundle of built in tool and exploits that helps for computer security. For the beginners it is a good start. I have uploaded the GNOME version of Backtrack. There is also a KDE version of backtrack in market. Backtrack is basically based on Ubuntu. There are a lot of software which are installed on Ubuntu to facilitate information security experts.
It contains network testing tools, packets sniffing and a lot of packs.