My issue is resolved.
Resolution:
1. Close all IE windows
2. Go to: \Program Files\Altiris\Symantec Installation Manager\Installs\Altiris\.. Here you should see files like ~ altiris_activitycenter_.... ..... .msi make a right click and uninstall (note if you can see two files like altiris_activitycenter_.... ..... .msi make uninstall both of them), then just move this file to another location.
3. Open cmd type: iisreset
4. Go to: \Program Files\Altiris\ rename the "ActivityCenter" folder to "ActivityCenter2"
5. Download the file from the link below and copy it to \Program Files\Altiris\Symantec Installation Manager\Installs\Altiris\
6. Run CMD with Admin rights and type:
msiexec.exe /i "C:\Program Files\Altiris\Symantec Installation Manager\Installs\altiris_activitycenter_7_1_sp2_x64.msi" skipaim=1
7. When this file will be installed go to:
\Program Files\Altiris\ActivityCenter\Web\Bin\ here must be a file - "Altiris.NS.Silverlight.ActivityCenter.Web.dll" right click >properties>details> If the Activity Center is successfully installed then here you must see file version 7.1.7580.0 (yes its version 7.1.7580.0 not the above mentioned by me)
8. In cmd run again iisreset then restart altiris service and clear browsing cache (from IE). This should have resolved the resolve.
You can download the file from here: http://www.sendspace.com/file/64hbsh