How to make IE10 default run in IE8 mode every time?

Go to the following path in your registry (run Registry Editor with the command regedit.exe from the Start Menu's Run command):

HKEY_CURRENT_USER\
  Software\
    Microsoft\
      Internet Explorer\
        Main\
          FeatureControl\
            FEATURE_BROWSER_EMULATION

Then...

1. Right-click on or in the folder and select New, then select DWORD (32-bit) Value.
2. Enter "iexplore.exe" (without quotes) for the Name and press Enter.
3. Double-click on the key (or press Enter). The Edit dialog opens.
4. Ensure the Base is set as Decimal.
5. Enter "9999" (without quotes) and click the OK button. Close Registry Editor.
6. Close all Internet Explorer windows.

IE will open by default as IE9