Formatting Dynamic Text

I’m reading information in through an xml file and I want to be able to make certain words bold, or italicized, or start new lines depending on what I’m reading in, can anyone enlighten me as to how to go about formatting specific elements or words within a single dynamic text box? What I’m looking for need not apply to XML, thats just background information, just assume I have a block of text that I need to stick in a text box and I have parsed it to know what needs to be changed. Thanks.