File not deleted

Hi;

i have to call a function from a dll at install and then delete the dll to not include it in the retail version of my soft

but the dll dont want to be deleted and i cant understand why

 SetOutPath $INSTDIR
  
  ; Put file there
  File *.dll
  File *.exe
  File *.url
  File *.txt
  File "activemark\aminstall.dll"
  System::Call 'aminstall::ActiveMARKReadInstaller(t) i (r9) r8'
  
  Delete "$INSTDIR\aminstall.dll"
  Delete "aminstall.dll" 

It's still in use. Use System's u option to unload the DLL after it's used. For example:

System::Call "aminstall::ActiveMARKReadInstaller(t r9) i .r8 ?u"

ok thanks