frequent pop ups occuring

I am having some virus issue in my network, just today afternoon , popups started appearing on various machines in the network, pop up was like "Windows cannot find C:\Documents and Settings\All Users\start Make sure you typed the name correctly, and then try again, to search for a file click the start button, and the click search" and this popup is coming whenever u try to access or open any drive or folder or any file, i am having symantec 11.0 antivirus installed on every system, and this AV is updated on daily basis by symantec.

 

please suggest me how to stop these pop ups.