Symantec AntiVirus, version 9.x is no longer supported, and was discontinued as of July 31, 2008. Virus Definitions are no longer tested against discontinued products, and compatibility issues (such as these) may occur.
There has been an Antivirus (AV) Engine Update posted 10 May 2010. Since this update, some legacy versions of SAV, namely 7.x, 8.x and 9.x are likely to be affected by the above symptoms. Due to the additional features added to the definition set, this would show an increase in size of the definition file. Because of this, it will affect old systems and some EOL products that may not work properly.
http://www.symantec.com/docs/TECH134713
In addition, older, unsupported versions of Symantec AntiVirus may not be able to protect your computer or environment against newer threats. As these threats are discovered, and designs are implemented to protect against them, they are included in the most current version of our products
http://www.symantec.com/docs/TECH103769