<!-- Hyperlink with default underline. -->
<Hyperlink NavigateUri="http://www.msn.com">
  MSN Home
</Hyperlink>

<Run Text=" | " />

<!-- Hyperlink with no underline. -->
<Hyperlink Name="myHyperlink" TextDecorations="None"
           MouseEnter="OnMouseEnter"
           MouseLeave="OnMouseLeave"
           NavigateUri="http://www.msn.com">
  My MSN
</Hyperlink>

// Display the underline on only the MouseEnter event.
private void OnMouseEnter(object sender, EventArgs e)
{
    myHyperlink.TextDecorations = TextDecorations.Underline;
}

// Remove the underline on the MouseLeave event.
private void OnMouseLeave(object sender, EventArgs e)
{
    myHyperlink.TextDecorations = null;
}