Windows

BCDEdit Command-Line Options

The boot.ini that was used on older Windows systems to control which operating system you boot into on dual boot systems is handled by BCD or Boot Configuration Data in Vista and Windows 7.
To edit the BCD you need to use BCDEdit.
BCDEdit Command-Line Options.
and here’s an FAQ with some nice examples.
http://technet.microsoft.com/en-us/library/cc721886(WS.10).aspx
For an example of how to use BCDEdit to boot windows 8 from a VHD file have a look here.
http://www.hyper-v.nu/archives/hvredevoort/2011/09/how-to-boot-from-vhd-with-windows-8-developer-preview/