Are you new to pc or do you know your way around? Basically you have the option to download the games with the gog download manager or download every file (if there are multiple files) separately. When you click on the game, turn the "gog.com downloaded mode" off (if you have difficulties with installer). But normally the downloader should work well for you.
It shouldn't really matter if you have win8 or not cause most games should work on it. There is also a 30 days guaranty, so if you really can't get the game to work, send the gog support team a ticket.
Also by clicking on community (above) and searching the game in 'Game specific forums', you will be redirected to the games' sub forum. You can ask your Q there too or use the search button to see if people had similar problems as you. By reading those threads you might find your answer :)