New Post: Cannot register task using standard user account

Hi all,
I'm reading this post since I'm having "Access is denied" on RegisterTaskDefinition, launching my application from a domain user which belongs to Administrators group on the machine (I'm trying on both Windows 7 and Windows Server 2008 R2).

What is strange is that thientan0206 said that the first operation works fine ("First, I create one task under administrator account like this one: [...] It's good, I can create the task, and let it run"). In my case, this simply operation already causes the "Access is denied" message error.

I tried to copy and paste these lines of code in a sample app:

using (TaskService ts = new TaskService()) 
{
                    TaskDefinition td = ts.NewTask();
                    DailyTrigger dailyTrig = new DailyTrigger();
                    dailyTrig.StartBoundary = DateTime.Now;
                    dailyTrig.DaysInterval = 2;
                    td.Triggers.Add(dailyTrig);
                    ExecAction ac = new ExecAction("D:\\a.exe");
                    td.Actions.Add(ac);
                    // Register the task in the root folder
                    ts.RootFolder.RegisterTaskDefinition("TEST", td, TaskCreation.CreateOrUpdate,
                    "SYSTEM", null, TaskLogonType.ServiceAccount);
}

...launched it and it doesn't work: I'm getting that annoying message.
Of course, if I try adding the same activity using the Task Scheduler snap-in, the activity gets created succesfully...
Why? What I'm doing wrong?