I'm not any knowledge on Vista, but one thing that takes my notice is your symantec corporate. Are you sure that does not include a firewall? I do know your model Linksys is 'not' a very reliable model. When you say you port forwarded, you mean you simply changed the settings in LW right, not actually port forwarded the Linksys?

* Try changing port in LW. You should connect using a different port to that of other computers on same network.

* Just to check, try connecting directly via your surfboard & not via the Linksys.