I have been witnessing a weird phenomenon with my computer.

I am running Windows 2000 Professional.
My system is protected through Norton Internet Security 2003.
I get connected to the Internet through LAN working under Microsoft Firewall Client.
While browsing the Internet, it very often happens that a strange warning dialogue box pops up on the screen announcing that the System is being shut down due to a command by NT AUTHORITY /SYSTEM. It further tells that windows/system32/isass.exe stopped responding under error status code 128. The system restarts within 60 seconds.

I wonder what might be wrong with my system as there is apparently no virus in that.

Could you please tell me how to rectify this problem?

this sounds like the blaster worm, except you've caught it about a year after the rest of the planet :)

have you downloaded all the critical updates for Win 2K? If not, get the one below

http://www.microsoft.com/downloads/details.aspx?FamilyID=c8b8a846-f541-4c15-8c9f-220354449117&displaylang=en

until you fix it, you can cancel the shutdown process by going to Start-->Run and typing shutdown -a

< insert profound, thought-provoking statement here >