Backtrack 5 is dedicated operating system for penetration testing. This is perfect for forensic. Download ISO from here.  
backtrack 5
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.
Download Backtrack 5