C# :: Flat Style Dropdown Menu Not Drawing Correctly In XP

I'm having issues with drawing a flat style dropdown menu in XP.

Here is the custom dropdown menu element:

public class FlattenCombo : ComboBox {
private Brush BorderBrush = new SolidBrush(SystemColors.WindowFrame);
private Brush ArrowBrush = new SolidBrush(SystemColors.ControlText);
private Brush DropButtonBrush = new SolidBrush(SystemColors.Control);
public Color HighlightColor { get; set; }

[Code] ....

Here is what it looks like in XP:

And here's what it should look like (this is how it's rendered in Windows Vista/7):