Phil, thanks for the response.

I created a custom action similar to that in the documentation that you
referred to and it worked.

the only drawback of the provided sample was that it used MessageBox api to
display an error to the user, which would show underneath of the main msi UI
because it has no parent window. since i found no way to obtain the parent
from the msi, I decided to use MsiProcessMessage to show my error message:

   PMSIHANDLE hrecord = MsiCreateRecord(1);
   if (hrecord == NULL || MsiRecordSetInteger(hrecord, 1, 25001) !=
ERROR_SUCCESS)
   {
       return ERROR_INSTALL_FAILURE;
   }
   MsiProcessMessage(hinstall,
INSTALLMESSAGE(INSTALLMESSAGE_ERROR|MB_OK|MB_ICONERROR), hrecord);

where 25001 is the ID of the error that i manually put into the Error table.

but this does not show any message box and simply follows the UI sequence as
if i called no MsiProcessMessage. the verbose log shows no message at all
also. looks like it cannot find the error message because deleting my error
row from the Error table makes no difference. why?

would appreciate any help!

konstantin