Hi,
If you are using the ACS software installed in a windows server, it would not have a serial number, you would ned to use the Sales order number for support.
Also please note, ACS 4.2 is EOL/EOS so the customer would need to migrate to a supported ACS version.
Here are the steps to migrate ACS 4.2 to the ACS 5.8 version.
http://www.cisco.com/c/en/us/td/docs/net_mgmt/cisco_secure_access_control_system/5-8/migration/guide/migration_guide/Migration_support.html
Thanks & Regards,
Kush