1) You can check SQL Configuration Manager->SQL Server Services->All services except SQL Server Browser are running.
2) If above step does not solve problem ,you can install Activity Center mannually to fix the activity center.
- Installed Activity Center msi using command -
C:\Program Files\Altiris\Symantec Installation Manager\Installs\Altiris >msiexec.exe /i "altiris_activitycenter.msi" skipaim=1
Note:You can put the correct msi name for activity center in above command that exists under Altiris Folder.
- Execute Altiris Deployment config file from NotificationServer->Bin folder –>
C:\Program Files\Altiris\Notification Server\Bin>AeXConfig.exe /configure "C:\Program Files\Altiris\Deployment\Config\Altiris.Deployment.config"