You may need a third-party program to convert them to a compatible format. The free Handbrake program is good for that.
What You Need To Do:
1. Plug your iPod into you PC and open iTunes.
2. Click on the iPod towards the top right hand side of the screen
3. Click on films
4. Tick the box marked sync films
5. Click apply and then sync
6. All films should then sync to iTunes and you should be able to watch them off of there.
7. To do this go towards the top left hand side of the screen and click on where it says music.
8. Change that to films and if you have synced them it should show all your purchased films.
Hope this helps