Lock window 7 via window service

Hi,  

I am trying to lock window 7 pc via a window service.  

I have been able to do so from win XP via code  

  Process.Start(@"C:\WINDOWS\system32\rundll32.exe", "user32.dll,LockWorkStation")  

 but when i trying this on Win 7 its not working.  

 Can anyone please provide me how i can lock Win 7 via window service or some alternate solution to it.