This article describes how to create a separate System partition and configure the boot environment to start both Windows Vista and Windows 7. The System partition is the disk partition marked as Active and contains the boot files used to start Windows. Windows Vista is usually installed on a disk partition that is marked Active.

Installing Windows 7 to a new partition on the same disk does not change the Active partition. However, creating a system image backup from Windows 7 includes critical boot files from the System partition, which may also include Windows Vista files. Using a disk configuration with a separate System partition for boot files supports a multiple boot configuration and allows a system image backup of Windows 7 without Windows Vista files.

Full tutorial on Technet.