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What Version of N.I.S. are you using, e.g. 2008?
I would suggest updating your Virus Definitions, check and make sure all boxes are ticked in the Manual Scanning section (N.I.S. tab > Settings > N.I.S. Options > Manual Scanning) and boot in to Safe Mode and do a Full System Scan.
If you have any questions about booting in Safe Mode, ask before booting.
Note: It could be that Windows has had a problem which requires a re-start or that Internet Explorer has had an Error and needs to Close; this is just to check that there are no Viruses on your computer.
Starting Windows Vista in Safe Mode
We suggest that you print this page before you start. PRINT
Because you must return to this Page after you re-start the computer, add this page to Favorites.

It can also happen if IE is not properly configured for Norton personal firewall.
Before trying the steps suggested by Floating_Red can you try the following.
Open Norton product>Click on NIS tab> Settings>Web browsing>Personal firewall>Configure.
In the new window,click on Program control under personal firewall.
In the Program control window, check for Internet Explorer.exe and change its status to Allow.
Same way change Microsoft generic host for win32 applications to Allow.
Apply and OK.
Now open IE ,Tools>Internet Options>Mangae Add ons>Enable or Disable
In the Add ons window disable Add on whose name does not make any sense.(Be careful while doing this, if you think its a useful one,please do not do this).
Now close and re open the IE and check .........