Merry,
Go to your Start menu and find the game's folder. Click on the folder to open it. One of the options should be "uninstall this game/demo".
Better yet, go get the free version of
Revo Uninstaller. All you have to do with that is open the program, right click on the name of the game/demo you want to get rid of, select uninstall and it will do it for you. All of your programs are automatically listed in Revo.
Just using the game's uninstall sometimes doesn't get rid of everything. Revo gives you the option of completely removing anything left behind.
Gil.