i opened several excel file, but it turns out to be opened in different excel
program, i.e. i cann't switch to the other excel files by pressing Ctrl +
Tab, as the files are opened by another excel. in order to switch inside
excel program i had to use open... from toolbar, instead of simply double
clip the file from explore.
i think there is one opions might have been set wrong, but just don't know
where i could reset this function.

thanks to reply me if you know this! it could be of great help to me.