ExecWait and run uninstall elevated

[dolu]

Hi all,
I'm trying to get an installer which uninstall previous version if needed.
So, everything is ok when the windows user is admin
But for a classical user, install part works well, but ExecWait never returns.

code:
...

!include "UAC.nsh"

...

RequestExecutionLevel admin

...

Function .onInit

	!insertmacro UAC_RunElevated

...	

 	MessageBox MB_OKCANCEL|MB_ICONEXCLAMATION "$msgAlreadyInstalled" IDOK uninst

  	Abort

	;Run the uninstaller

	uninst:	

  		ClearErrors

  		ExecWait '$R0 _?=$INSTDIR' 

  		IfErrors no_remove_uninstaller done

  	no_remove_uninstaller:

 ...

	done:

FunctionEnd

...

What's wrong ?
Is there another way to call the uninstaller inside the installer ?
Thanks.