I installed Uplay from Ubisoft and installed Assassins Creed 2 on my new Windows 10 PC. When I started the game I got two error messages which are related to the DirectX, here is how I solved it.
1.
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AssassinsCreedIIGame.exe - System Error
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The program can't start because d3dx9_42.dll is missing from your computer. Try reinstalling the program to fix this problem.
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OK
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2.
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AssassinsCreedIIGame.exe - System Error
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The program can't start because XINPUT1_3.dll is missing from your computer. Try reinstalling the program to fix this problem.
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OK
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The first message is showing D3dx9, so I downloaded and tried to install the DirectX, but it showed the following error
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DirectX Setup
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An internal system error occurred.
Please refer to DXError.log and DirectX.log in your Windows folder to determine problem.
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OK
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Later I downloaded the Web installer for the DirectX and installed it. That fixed all the issues. You can get the installer from here.