If you are using a dual-boot with more than one operating system, it is fairly easy to configure the default operating system with graphical interface. Note that if you use Windows 7 or Vista as an operating system, you have to make this change from 7/Vista, but not XP.

First, you will require right clicking on Computer and selecting Properties:

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Next, click Advanced System Settings

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Now click on the Settings button under Startup and Recovery

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And just choose the OS you want to use:

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You can also use the command Bcdedit do the same thing from the command line, but it is probably easier that way.