C:\Program Files\Common Files\Symantec Shared\VirusDefs folder takes more than 1.58 GB .
are there tmpXXXX.tmp files under this folder?
If yes, can you check the steps
Configuring the number of content revisions kept by the Symantec Endpoint Protection (SEP) client 11.x
Help Link: http://www.symantec.com/business/support/overview.jsp?pid=54619
you can delete all the old date definition..
Configure the Disk space Management for Virus Def.
To configure the disk space used for LiveUpdate downloads
In the console, click Admin.
Click Servers and select the site that you want to configure.
Under Tasks, click Edit Site Properties, and then click LiveUpdate.
Under Disk Space Management for Downloads, type the number of content downloads that you want to store.
If you want to reduce the amount of disk space used, uncheck the Store client packages unzipped to provide better network performance for upgrades option. Note: Disabling this option also disables the ability of Symantec Endpoint Protection Manager to construct deltas between content revisions and may adversely affect network performance for updates.
Disk Space Management procedures for the Symantec Endpoint Protection Manager
Drive Space used by Virus Definitions Updates
MIM | MCSA | MCTS | STS | SSE | SSE+ | ITIL v3
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What is the version of the product?
Is it Symantec Antivirus 10.x, Symantec Endpoint Protection 11.x or 12.1?
What updates (service packs and hotfixes) are released for Backup Exec 2012 version?
kindly update the latest status of your issue?
If you see more defination being stored..... that would be cause the definations are corrupted.
I would suggest you to stop the smc services of the client and delete the definations and then monitor the issue. I am sure this would help.
From this locaiton that you have provided about virus defs.... >> C:\Program Files\Common Files\Symantec Shared\VirusDefs
This is the locaiton for 11.0.x client.
You could also refer to below article
Article URL http://www.symantec.com/docs/TECH141811
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