At times you may way to use your own code to schedule a task through power shell, that can be done by using the following powershell script task:
$compname = "%COMPNAME%"
$URI = "http://localhost/Altiris/ASDK.Task/TaskManagementService.asmx"
$proxy = New-WebServiceProxy -Uri $URI -Class Task -Namespace webservice -UseDefaultCredential
$inputXml = @"
<inputParameters>
<parameter>
<name>@AssignedResources</name>
<value>%COMPID%</value>
</parameter>
<parameter>
<name>@CompRequirements</name>
<value>
<minWaitTime>1 minutes</minWaitTime>
<maxWaitTime>10 minutes</maxWaitTime>
<minCompletion>95 %</minCompletion>
</value>
</parameter>
<parameter>
<name>@OverrideMaintenanceWindows</name>
<value>
<boolean>true</boolean>
</value>
</parameter>
</inputParameters>
"@
$proxy.ExecuteTask("15f4a911-63f6-4132-bf28-d7c0081f7625" ,"PowerShell Execute on $compname", $inputXML)
See the Altiris Task ASDK for help and details