Symantec's Norton software, like Norton Antivirus, Norton Internet Security, or Norton 360, is notorious for causing networking problems (see: Not enough server storage for one example). It is so bad that even uninstalling it may leave the computer's network installation permanently damaged, apart from leaving lots of stuff on your computer, instead of uninstalling it.

To alleviate this problem, Symantec has been forced to create a separate cleaning program, named Norton Removal Tool, which can be downloaded on this page: http://service1.symantec.com/support/norton360.nsf/docid/2006112909122875

Even this removal tool will not repair all network damage done by the Norton software, but it's a step in the right direction. You should use it.