This tutorial will show you how to pick which system you want to start up your comp with. Especially if u have dual booted and windows keeps booting up instead of your Mac os.
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Step 1: Click System Preferences from Dock or Apple Taskbar

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Step 2: Select Startup Disk from Preferences Menu.

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Step 3: Choose which system you want to start your computer up with. Originally its supposed to be Mac OS X, Mac OS X HD, Macintosh etc.
I have dual booted Windows 7 Ultimate with MAC OS X So I use a boot loader instead
of this feature so I don't have a MAC OS X, etc icon there. I just have the Network System.
But lets just pretend that is my Mac OS X OS.

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Step 4: If you have already picked the System -> Choose Restart

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Step 5: Confirmation will come up to Restart -> Click restart.

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Step 6: Check if the system you picked booted up, if not Redo this whole process atleast one more time, or post questions here