The best tool int he universe is Penetration Testing Linux. Most common released as ISO. You need to burn CD and boot you PC from CD.
Linux in fully armed with ALL possible scanning, enumeration and hacking tools. There is a fancy GUI, all tools are sorted and zero Linux knowledge needed. NMAP, Nessus client and server, Metasploit are included.
The most popular editions are
Whax and
Auditor. Both teams joined forces and Finlay 1 week ago the new distro was released -
BackTrack P.S. With "MySlax Creator" you can install BackTrack to 1Gb USB drive. The pen test tool will boot in seconds. Original CD version boots pretty fast also.