MR3 for Symantec End Point Security 11.

There have been a couple of KBs around knows issues with Symantec End Point Security 11 like:

You may experience various problems when you work with files over the network on a Windows Server 2003-based or Windows 2000 Server-based computer
https://support.microsoft.com/default.aspx?scid=kb;EN-US;923360

And
You experience slow file server performance and delays occur when you work with files that are located on a file server
https://support.microsoft.com/default.aspx?scid=kb;EN-US;822219

I also blogged about long time ago here.

Now there is one more KB regarding the same;

Unable to access Shares "The specified network name is no longer available".
https://support.microsoft.com/kb/961293

Here are couple of heads up;

The link in that KB, https://www.symantec.com/business/support/endpointsecurity/sep11_faq_customer-installations-issues-with-resolutions_int_112007%20partner(3)).pdf 
is dead!

Here is the right link:
Symantec Endpoint Protection 11.0—Customer Installation Issues with Resolutions
https://www.symantec.com/business/support/endpointsecurity/customer_installations_issues_with_resolutions_int_112007_partner.pdf
(This might change again, since Symantec would keep updating it. Simply search for “Symantec Endpoint Protection 11.0—Customer Installation Issues with Resolutions” to get the latest link.)

Secondly the KB does not talk about MR3 update, it talks about MR1. And IMHO, before you uninstall Symantec End Point Security 11, try installing MR3 update and it may resolve all know issues. You could also try disabling it to identify if it is the cause.

Let’s not uninstall an investment in these difficult times…