What are the advantages and disadvantages to using standard library calls vs the OS's API calls when programming something you know will be targeted for one operating system?

For example, in C++ I'm programming specifically for Windows, so should I do this

```
memcpy(studentCopy, student, sizeof(Student));

```

Or stick to Windows API calls, as such:

```
CopyMemory(studentCopy, student, sizeof(Student));

```

I'm unsure of which convention is better and have been unable to find anything from searches. Is it simply preference?

Edit: The student thing was just an example of using different libraries, not asking about that specifically, it was the first thing that came to mind.