|A lot of laptop manufacturers include a System Restore facility on their machines, and this lets you boot into an environment where you can reset the system to its original configuration.|
This is often put into a separate partition (i.e. the D drive) to prevent users from accidentally copying over it.
You can overwrite it, but will lose the restore facility if you do. Depends on what you're backing up to DVDs.......just data or the entire system?