<p>I want to diaplay message boxes that looks good, with the possibillity to use bold/italic text and text with different colors. The standard MessageBox.Show doesn't provide this functionality.</p>

<p>I have been trying to create a message box using the WebBrowser control to display nicely formatted text using HTML-syntax. But I haven't succeeded.</p>

<p>Do you know if this is possible, or do you know any other way to show a message box with formatted text? I want the message box to size itself dynamically to it's content.</p>

<p>(For Windows Forms)</p>

You could create your own Form class that hosts the WebBrowser control (or an RTF control) to display the formatted content. Just give it an interface (API) similar to MessageBox so you can use it much the same way (e.g., checking the user's Yes/No or Ok/Cancel/etc responses).
For the sizing, you would likely need to check the content being displayed and use some System.Drawing capabilities to get the size of the fonts/content for it. Then size your window accordingly and/or show and hide scrollbars so they're only shown when absolutely necessary.