Making specific Text Boldefaced in a TextBox

use a RichTextBox, below a method that i have wrote for this problem - hope it helps ;-)

/// <summary>
/// This method highlights the assigned text with the specified color.
/// </summary>
/// <param name="textToMark">The text to be marked.</param>
/// <param name="color">The new Backgroundcolor.</param>
/// <param name="richTextBox">The RichTextBox.</param>
/// <param name="startIndex">The zero-based starting caracter position.</param>
public static void ChangeTextcolor(string textToMark, Color color, RichTextBox richTextBox, int startIndex)
{
    if (startIndex < 0 || startIndex > textToMark.Length-1) startIndex = 0;

    System.Drawing.Font newFont = new Font("Verdana", 10f, System.Drawing.FontStyle.Bold, System.Drawing.GraphicsUnit.Point, 178, false);
    try
    {               
        foreach (string line in richTextBox.Lines)
        { 
            if (line.Contains(textToMark))
            {
                richTextBox.Select(startIndex, line.Length);
                richTextBox.SelectionBackColor = color;
            }
            startIndex += line.Length +1;
        }
    }
    catch
    { }
}

You can use TextBlock with other TextBlocks or Runs inside:

<TextBlock>
    normal text
    <TextBlock FontWeight="Bold">bold text</TextBlock>
    more normal text
    <Run FontWeight="Bold">more bold text</Run>
</TextBlock>

You will need to use a RichTextBox to achieve this:

<RichTextBox Name="richTB">
  <FlowDocument>
    <Paragraph>
      <Run FontWeight="Bold">Your Name:</Run>
      <Run Text="{Binding NameProperty}"/>
    </Paragraph>
  </FlowDocument>
</RichTextBox>

But why would you want "Your Name" to be editable? Surely you would want it as a separate, readonly, label?

<StackPanel Orientation="Horizontal">
    <Label FontWeight="Bold">Your Name:</Label>
    <TextBox Text="{Binding NameProperty}"/>
</StackPanel>