You can run giac without installing it by using a javascript-enabled web browser. Using Firefox for this is highly recommended; Firefox runs giac several times faster than Chrome, for example, and Firefox also supports MathML natively.
To run giac in Firefox, open the url https://www-fourier.ujf-grenoble.fr/~parisse/giac/xcasen.html. At the top of this page will be a button which will open a quick tutorial; the tutorial also tells you how to install the necessary files to run giac through Firefox without being connected to the Internet.