Vdownloader cannot Perform

I have download Vdownloader an hour earlier & it work perfectly O.K.
But for some reasons, I un-installed it. However, within an hour, I re-installed Vdownloader in exactly the same manner & this time, although the Vdownloader is installed, but when I try to copy the video (unprotected video), the Vdownloader keeps prompt me download error. I have un-installed the program & reinstalled it again, but it is still the same. The note below Vdownloader wrote "..... because it is being used by another process ".

I suspect there might be some old residual program, what shall I do ?