Please look at "Software feature activation keys", under the section Cisco TAC Resources in the document at "http://www.cisco.com/en/US/products/sw/secursw/ps2120/prod_troubleshooting_guide09186a00801b83bf.html". Basically, you need to log in, fill in the form with the required info and submit the same. Information you need to keep ready includes things such as your identification and the PIX serial number.
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