changing a MUI control by application

changing a MUI control by application
I have to create a custom page which displays some checkboxes, execute some file operation (copy, mkdir, del,..) and checks the checkboxes depending on the results of the file operations.
Not the user should check the checkboxes!!!

I tried a lot with sendMessage and so on but I didn't found a solution. I'm only able to check/uncheck the boxes before I show the dialog. Once the dialog is displayed I can't let my function change the controls.

Is there a way to do this?

Here some code from my test app:

...

!insertmacro MUI_PAGE_WELCOME
Page custom MyDialog
!insertmacro MUI_PAGE_FINISH

...

Function .onInit
!insertmacro MUI_INSTALLOPTIONS_EXTRACT "MyDialog.ini"
FunctionEnd

...

Function MyDialog
!define BM_SETCHECK 0x00F1
!define BST_CHECKED 0x0001

!insertmacro MUI_INSTALLOPTIONS_INITDIALOG "MyDialog.ini"
!insertmacro MUI_INSTALLOPTIONS_SHOW

Pop $HWND
GetDlgItem $DLGITEM $HWND 1201 ;1st checkbox
SendMessage $DLGITEM ${BM_SETCHECK} ${BST_CHECKED} ""

!insertmacro MUI_INSTALLOPTIONS_SHOW

;[ -- here should be some file op's -- ]

GetDlgItem $DLGITEM $HWND 1202 ;2nd checkbox
SendMessage $DLGITEM ${BM_SETCHECK} ${BST_CHECKED} ""

FunctionEnd

The first checkbox is checked correctly (but before _SHOW).
But I want that the second checkbox is checked right after executing my file op's.

Please help ...

thanks