Hi Zora-Link, sorry this is affecting your enjoyment of the game. Have you tried lowering the default graphics settings (Main menu > Settings > Graphics), particularly turning off shadows and after effects?
Can you also post a DxDiag?
Windows XP: Go to your start menu and click on run. Type in "dxdiag" and hit enter.
Windows Vista / 7: Press the windows key on your keyboard or click on the start menu. Type in "dxdiag" and hit enter.
Now click on the button that says "save all information".
It will prompt you to save a file. Save it where you can find it.
Open the file and press Ctrl+A followed by Ctrl+C to copy the contents, and paste them in a reply by pressing Ctrl-V.