Function .onInit

         Exec $INSTDIRuninst.exe

FunctionEnd

; The "" makes the section hidden.
Section "" SecUninstallPrevious

    Call UninstallPrevious

SectionEnd

Function UninstallPrevious

    ; Check for uninstaller.
    ReadRegStr $R0 HKLM "${HKLM_REG_KEY}" "InstallDir"

    ${If} $R0 == ""
        Goto Done
    ${EndIf}

    DetailPrint "Removing previous installation."

    ; Run the uninstaller silently.
    ExecWait '"$INSTDIRUninstall.exe /S"'

    Done:

FunctionEnd