Hi Noxx, demos for Supremacy have been out for quite a while. Mike updates them on a regular basis - in fact the latest version was only released 2 weeks ago, and it fixes an issue with planet and star densities and adds in animations to map objects like Stars.
Mike ALWAYS puts the download in the exact same place - in the first post of the Supremacy Pre-release download thread, which you can find
I'm not giving you the exact download link because you really do need to read the information in the post - AND follow it - before you play Supremacy. Supremacy needs certain Windows components and addons to be installed and fully up-to-date before you can play the game. Links to these necessary downloads are contained in the post for you.
You need these components because Supremacy is based on a lot of Vista technologies - allowing for amazing special effects and greater moddability. Don't worry though, these technologies ARE backwards compatible to XP.