Hello,
This issue could be due to various reasons:
1) The Package downloaded itself is not a complete package.
i.e; Verify the file Downloaded size of the .zip file
(Symantec_Endpoint_Protection_11.0.6300.803_RU6_MP3_Xplat_EN.zip is of 1.35 GB (1,456,665,873 bytes) )
2) You are running the Setup directly from the .zip file downloaded, make sure you unzip the File and then run the setup.exe
3) Make sure you are not running the Setup from a Mapped Shared folder / drive.
4) Check the steps from Microsoft.
http://support.microsoft.com/kb/314810