I think virus definitions got corrupted in you pc
Pls refer the following doc
How to clear out corrupted definitions for a Symantec Endpoint Protection Client manually.
http://service1.symantec.com/SUPPORT/ent-security.nsf/docid/2007123111551948
Since this is an old version and a lot of fixes are present in new versions you may also consider the upgradation to RU5(latest available version) also..