1.Boot with Win2008 installation DVD.
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2.select repair menu
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3.select command line from repair
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4.Type "Bootrec /Scanos" to scan the os without listing in boot sequnce.
5.Type "Bootrec /RebuildBCD" to rebuild BCD record files in the first partition of first disk.
6.Type "Bootrec /Fixboot" to add a new record in BCD
7.Type "Bootrec /Fixmbr" to fix the mbr.
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8.Then reboot your machine.
9. If the doul boot menu will be not reproducible,you can copy "bootmgr" in your Win2008 installation DVD to your first partition of first disk.
10.Reboot your machine.
You will find a your vista boot menu.
According to the article as belows,you will find the answer lies in the questions.