If you have too many cores then Assassin's Creed will not start.
The solution is pretty simple and straight forward:

We will create a batch file which will start the game with certain cores only.

• In your Steam library right-click "Assassin's Creed" and select "Properties".
  
• Left-Click "Installed Files"
  
• Left-Click "Browse..."
  
• Now you are in the correct folder and can use it directly or copy/paste the path and use your normal way of doing things.

Create a file with the name "start-affinity.cmd" with this content:

Yes, you read right. "testapp.exe", not something else, this is NOT an error.

If this is not working, try changing the hexmask.
Here is an Url[bitsum.com] which can quickly help you, but if you want to understand what you are doing then open a command line and enter:
and look for "AFFINITY" to get started.

• Double left-click the .cmd file
  
• If Assassin's Creed starts, we are nearly done with the the first half of our fixes.
  
• With the RIGHT mouse button, drag&drop the .cmd to your desktop.
  
• Select "Create shortcut here".
  
• Rename the shortcut to "Assassin's Creed".

If you are feeling fancy, change the icon of the desktop shortcut:

• Right-click the shortcut on your desktop
  
• Properties
  
• Shortcut
  
• "Change Icon..."
  
• Browse to the directory where you just created the .cmd
  
• Select one of the .exe files which look like assassin's heads
  
• select the icon

The reason for the stuttering is that the game is trying to connect to a server which does not exist any longer.
We can ease that pain.

• Open "notepad" or some like that as administrator.
  
• Then open "C:\Windows\System32\drivers\etc\hosts" and add these lines:
  
  203.132.26.155 127.0.0.1 216.98.50.240 127.0.0.1
• Save the file

Now the game should not stutter any more.

I did not need the 2nd line when I created this guide. But you can read the comments, someone (Rob) said he needed it now. So I added the 2nd line.