System partition and boot partition
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System Partition
In Windows computing the Active Partition is another name for the System Partition.
The system partition is the partition that contains a boot sector pointer to the following 3 files.
Boot.ini Ntldr NTdetect.com
Note: This partition is can be different from the Boot Partition although they are often on the same partition. The Windows installer places the initial system partition based on motherboard bios settings.
See also Partition (computing)
References
http://support.microsoft.com/?scid=http%3a%2f%2fwww.support.microsoft.com%2fkb%2f100525%2fen-us%2f