What about
<CustomAction Id="TaskCreate" Return="check" Execute="deferred" Directory="TARGETDIR" Impersonate="no"
ExeCommand="SCHTASKS.EXE /CREATE /TN "My Client Service" /RU SYSTEM /RP /F
/XML "[ProgramFilesFolder]\MyProduct\Client_Task.xml"" />
[ProgramFilesFolder] is C:\Program Files for x86 machine and C:\Program Files (x86) for x64 mashine if you are making a x86 package.