\documentclass{minimal}
\usepackage{fontspec}
\begin{document}
%\setmainfont[Ligatures=TeX]{texgyretermes-regular.otf}
\fontsize{25bp}{35bp}\selectfont
\parindent=0pt

%OpenType Termes is included in TeX Live.

\noindent Temporarily switch to {%
\fontspec[Mapping=tex-text]{Comic Sans MS}
Comic Sans MS} and to {%
\fontspec[Mapping=tex-text]{Tahoma}
Tahoma} and back.

\end{document}
