Hello,
If your iPod isn't jailbroken and the movie was rented from the iTunes Store, it is very unlikely that you have a virus.
Have you tried exiting out of running programs? Leaving multiple programs running can slow down your device.
You can do this by double tapping the home button, a row of apps will appear. These are the apps that are still running in the background. You will then need to hold down one of the apps until a red circle with a minus symbol appears in the top corner (may also be an 'x' in a black circle, depending on iOS version). You will need to tap the red circle for each of these apps.
Do this regularly to prevent your device from lagging.
Regards,
Jamie
