After installing Windows 7 as a dual-boot setup, it’s automatically set as the default boot option (shown at startup). The default Operating System starts automatically after the specified timeout period elapses. If you want to unset Windows 7 as the default boot option, use the System Configuration Utility in Windows.
Start Windows (Windows 7, Vista or XP)
Click Start, type MSConfig.exe and press ENTER
Click the Boot.ini tab
Select an Operating System and click Set as default
Note that this is not specific to Windows 7, but also applies to Windows 2000/XP/Vista.
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Ramesh Srinivasan founded Winhelponline.com back in 2005. He is passionate about Microsoft technologies and has a vast experience in the ITeS industry — delivering support for Microsoft's consumer products. He has been a Microsoft MVP [2003 to 2012] who contributes to various Windows support forums.