WM6 is newest operating system (Windows Mobile 6) for the Treo 750. If you have ATT you can get it here for free:
http://www.palm.com/us/support/downloads/treo750.html
if you dont have ATT I am not sure where to get it.
Running videos on the 750 you need a lot of free space.
Press Start, setting, go to the system tab at the bottom and click on memory. I usually need 10-15 megs free to play videos.
If you dont have that, do a soft reset by pressing the button inside the minisd card door. After restarting see what you have and if still not much you probably have to many things installed.
Some themes, weather stuff and other things dont always show up as running programs and are hard to stop. I use Weatherbug for weather and its like that no way to stop it except to uninstall.