Since laptops can rarely host more than on drive, we'll assume that this is a single drive partitioned into two logical drives.
Many (most, actually) laptops are partitioned from the factory. There is usually a hidden diagnostic partition before what's called the C drive. As long as this partition remains hidden, no big whoop.
Unfortunately, reinstalling Windows manually will often unhide the hidden partition. Windows will always put bootfiles on the *first* available partition of a drive. Again, unfortunately, the tiny hidden partition lacks the room to host Windows, but it can host the bootfiles.
So, if someone reinstalls Windows manually on these notebooks, they will have no choice but to install to the larger D partition.
By the way, this was a common layout in business PCs running NT in the 90s. There was usually a small 10MB C partition with the bootfiles and the rest was allocated as D and had Windows and all other programs. I'm never surprised when I see this type of setup.
As Steve says, you can't change this. If you hide the C drive, it will be the last thing you do before having to reinstall Windows because it won't boot.
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