Task Scheduled and video files

I created a task with the Task Scheduler to run a wma file in WMP every half an hour.
How can I prevent it from popping up when I am actually watching a video in WMP or Nero, or another video program?

The workaround is to create the task wiith privileges of another user than the one that is currently logged on. You may use a system user like 'NT AUTHORITY\SYSTEM'.

Thanks for your help.
I am not an expert. Can you explain your last sentence?
God bless

OK. Open task scheduler and double click on your particular task.
In the properties window that opens up, under "Security Options", click on "Change User or Group" (With an earlier version of Windows, it might be "Run as").
In the empty box with the blinking cursor ("Enter object...."), type "SYSTEM" (without the quotes). Then click on "Check Names", it'll show NTAuthority/System. Click OK, OK.
I also have "Run whether user is logged on or not" and "Run with the highest privileges" selected.