When the computer is starting keep tapping the F8 key. You will see a menu, choose SAFE MODE.
This should not load Norton and you should be able to use your computer. When your in safe mode it will look horrible but here you can sort the mess out.
First i would stop Norton from running. Go to start > Run > Type:
Click ok. Now up at the top click on STARTUP remove the tick from the box beside Norton AntiVirus.
Restart the computer and see if you can boot into NORMAL windows. If you can i would suggest you remove norton from the computer via Add/Remove in the control panel.
This article explains how to remove all traces (nearly all traces) of NAV from your computer click here
Then when done reinstall.
Also what Operating system are you using?
What is the specification of your PC?
When you say " I loaded an updated version of Norton AV" do you mean you updated the Virus definition files or you updated the whole program? For example NAV 2003 to NAV 2004?