Yes the problem occurs because u have set both height and width to your textbox. Set either one of them. The reason being Scrollviewer helps to scroll only when the height or width required by the textBlock/control is not available and then it provides scroll bar to see the content. since the height and width of the textbox(since explicitly set )is within the range of the scroll viewer, it doesnt provible scrollbar. Most of the people like to have vertical scroll bar hence remove height from text box and now try
the code:
<ScrollViewer VerticalScrollBarVisibility="Auto" Height="62" Width="330" Margin="28,263,291,135">
<TextBlock Name="about" TextWrapping="WrapWithOverflow" Text="sth" Width="330" />