Okay, basically in a monitor when it refreshes to display a new picture, it is cycling through all the pixels from top to bottom, changing the image.
Your monitor, 100HZ means that it is doing this 100 times a second.
This is also why if you were to take a picture of your monitor, you'll see the image but they'll be black lines down it.
So vsync just works with the video card to make sure that the card sends out an image exactly as the monitor refreshes. Instead of it rendering as fast as it can loads of images, and them displaying on the monitor when it is refreshing (which can lead to a thing called banding (or something) where the screen flickers slightly as it is sending images because the monitor might be half way through its refresh as it sends out a new one to the monitor.
This on yours should be no problem, and you can disable it no probs, and it might just fix it.
As for the cinematics, I' am pretty sure it'll be becaause it is running them from the cd/dvd instead of the hard drive. It can't be anything else
EDIT: oh... hmm, so it used to use the cd just to see if it was there to start playing? But the crack disabled it doing this?
I still wouldn't think it'd be your card though, and maybe the step above will help (You might find in some games, menus will work faster too, as some games are pre-programmed with time limits. e.g. need for speed underground)
+ When you say they are slow, do you mean it takes a longer for the image to change? Or that it actually runs slow? (slo-mo)
I hope this clears up some questions