Hi KS,
See below this will help u to understand the problem and resoliution
first check above thraed which james has provided as there may be also IE issue. also refer below
https://www-secure.symantec.com/connect/downloads/remote-altiris-agent-diagnostics-20-raadv2
https://www-secure.symantec.com/connect/forums/altiris-agent-not-installing
http://www.altirigos.com/forums/threads/altiris-agent-not-communicating-with-ns-server.11574/