This code may help you:
String[] dirs = System.IO.Directory.GetDirectories("C:\\");
int i;
for (i = 0; i < dirs.Length; i++)
{
listBox1.Items.Add(dirs[i]);
}
String[] files = System.IO.Directory.GetFiles("C:\\");
for (i = 0; i < files.Length; i++)
{
listBox1.Items.Add(files[i]);
}