Go to Software Download, Cisco Secure Software, chose PIX.
Download the file: asdm-demo-521.msi
Cisco Adaptive Security Device Manager demo image.
This lets test the GUI of a ASA 5520, looks and feels a lot like PIX.
http://www.cisco.com/cgi-bin/tablebuild.pl/pix
IF you really want to learn how to configure PIX and ASA, consider a classroom learning, e.g. by GlobalKnowledge. The training is named: SNPA (Securing Networks with PIX and ASA)
http://www.globalknowledge.com/training/cisco.htm