I use
chartRange = xlWorkSheet.Rows[1];
chartRange.Font.Bold = true;
to turn the first-row-cells-font into bold. And it works, and I am using also Excel 2007.
You can call in VBA directly
ActiveCell.Font.Bold = True
With this code I create a timestamp in the active cell, with bold font and yellow background
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
ActiveCell.Value = Now()
ActiveCell.Font.Bold = True
ActiveCell.Interior.ColorIndex = 6
End Sub