Yes: the simplest way is to handle the Form.Shown event and add this:
string[] files = Directory.GetFiles(@"C:\MyProjects\", "*.*", SearchOption.AllDirectories);
myComboBox.Items.AddRange(files);
That adds the entire path, so if you want to add just the files names you will have to use a little Linq method:
string[] files = Directory.GetFiles(@"C:\MyProjects\", "*.*", SearchOption.AllDirectories);
var namesOnly = files.Select(f => Path.GetFileNameWithoutExtension(f)).ToArray();
myComboBox.Items.AddRange(namesOnly);