hi
linux can not play hd movies as well as windows....is not quite descriptive enough to research the errors.
could you post the errors you see in attempting to play the movie....or is that you have patchy sound or jerking video?...got the idea?
2) since you mention Debian, its possible you are using the open source vidoe card driver and not the closed source driver.
are you against trying the closed source driver from nvidia?
3) As you mention root and non-root user....its is always safer to play multimedia as a normal user, but your distro may not have set up the appropiate permissions for you?
Since I do not know your errors as yet...forgive me for mentioning you need to be a member of the video and audio groups
to check run...as a normal user
you may get something like
YOURname dialout cdrom floppy audio dip video plugdev users fuse scanner netdev powerdev