<p>What is the best way to <strong>display</strong> and <strong>print</strong> XPS files in a Windows Forms application?</p>

I think the easiest way is to use WPF, you can host a WPF control inside a WinForms application - you don't have to rewrite your GUI in WPF or anything like that.
more information here :
http://msdn.microsoft.com/en-us/library/ms745781.aspx