If you downloaded the game a few months ago and are only starting to play it now, there have been patches, and you might want to redownload it. If you downloaded the game recently, the download should already be patched.
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Without knowing more about your computer, any suggestions I'd have are only guesses about what could be causing the problem. However...
1. What antivirus are you using?
Windows 8 comes with its own antivirus. If you've added another one, make sure the computer is only using one of them or you increase the chance of freezes and crashes -- especially when playing games. Also, certain antivirus (like McAfee) have been known to interfere with games all by themselves. But if you're only using the antivirus that comes with Windows 8 (Windows Defender), the antivirus is probably not the problem.
2. Make sure your computer's video card meets the system requirements -- especially for the video card. These are the listed video card requirements listed at
***GOG*** and
***Steam***:
Graphics: ATI or NVidia with 512 MB RAM
If you're using Intel graphics or have less than 512 MB of video memory, that might cause a problem.
3. Are you using a laptop? If so, there could be problems because laptop graphics normally use different graphics drivers from desktops. There are also laptops that use graphics that automatically shift to a low-power-graphics mode. Unfortunately that can cause problems with adventure games, which normally use less power than action games and aren't detected properly.
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You can probably get better help from Phoenix Online (the developer that created the game)
Phoenix Online has a forum specially devoted to technical support for GK 20th here
http://www.postudios.com/blog/forum/index.php?board=132.0They also have a thread about how to submit technical support requests and what information they need from you here
http://www.postudios.com/blog/forum/index.php?topic=14157.0