I've never seen a laptop which automatically moves data once one drive is full.
What's probably going on in a laptop (and this is just an educated guess) is that you actually have just one hard drive, which is divided into two parts or "partitions". In this case, as far as the laptop is concerned, you have two drives, even though physically you only have one.
Probably what the manufacturer intended is for you to use the C: drive only for the operating system (Vista) and for programs, while you save stuff in the D: drive. Whenever you're asked to save something, you should be given the choice of where to save it. If you want to save things to the D: drive, you can just click on "My Computer" when you're asked where you want to save something, and click on the D: drive from there.
If your C: drive starts running out of space, your computer should tell you when there isn't enough room to save something else. It will probably also start running a little slower as the drive starts to fill up if its the same drive as the hard drive, so its usually good to save things on a different drive.
Hope that helps you out!