Did you use this Norton uninstaller tool http://us.norton.com/support/kb/web_view.jsp?wv_type=public_web&docurl=20080710133834EN
, then reboot your machine afterwards?

Also, I’d check while in Safe Mode for remnants of Norton/Symantec under Search to see if there is anything left behind, as there often is…and this would create a conflict with trying to install a new AV. If anything is found left over, delete and reboot. Then do a cleaning with something like CCleaner Slim version http://www.piriform.com/ccleaner/builds and reboot again.

What other security software are you using (firewall, trial security software, other security software)?