What you need is to specify the right combination of window styles, WPF does not expose all the options available in Windows but you can set them yourself using pinvoke.
I'm not at a Vista machine right now so I can't test style combination to see what gives the correct look but the list of styles (in C#) is here http://pinvoke.net/default.aspx/user32/GetWindowLong.html
in you're Window class:
[DllImport("user32.dll")]
static extern int SetWindowLong(IntPtr hWnd, int nIndex, uint dwNewLong);
[DllImport("user32.dll")]
static extern bool SetWindowPos(IntPtr hWnd, IntPtr hWndInsertAfter, int X, int Y, int cx, int cy, uint uFlags);
private const int GWL_STYLE = -16;
private const int GWL_EXSTYLE = -20;
private const UInt32 SWP_NOSIZE = 0x0001;
private const UInt32 SWP_NOMOVE = 0x0002;
private const UInt32 SWP_NOZORDER = 0x0004;
private const UInt32 SWP_NOREDRAW = 0x0008;
private const UInt32 SWP_NOACTIVATE = 0x0010;
private const UInt32 SWP_FRAMECHANGED = 0x0020;
public override void OnSourceInitialized(EventArgs e)
{
IntPtr hwnd = new System.Windows.Interop.WindowInteropHelper(this).Handle;
// set styles to a combination of WS_ flags and exstyles to a combination of WS_EX_ flags
SetWindowLong(hwnd, GWL_STYLE, styles);
SetWindowLong(hwnd, GWL_EXSTYLE, exstyles);
// and to activate changes:
SetWindowPos(hwnd,IntPtr.Zero,0,0,0,0,SWP_NOSIZE|SWP_NOMOVE|SWP_NOZORDER|SWP_NOACTIVATE|SWP_FRAMECHANGED);
}