It's been a while since I used XGL, but this should work.

1. Logoff
2. Switch to a text console; ctrl-alt-f1
3. Login as root
4. Enter the commands:

telinit 3
gnome-xgl-switch --disable-xgl
telinit 5 ; exit

5. Login to the GUI which should appear after the last command above
6. Watch the movie

To switch back to XGL:

1. Logoff
2. Switch to a text console; ctrl-alt-f1
3. Login as root
4. Enter the commands:

telinit 3
gnome-xgl-switch --enable-xgl --auto
telinit 5 ; exit

5. Login to the GUI which should appear after the last command above