I bought a computer that runs Windows 7. The HDD contains three Partitions: C, D and a System Reserved partition without a drive letter.
As default, the C:\ drive is the boot directory that contains the Windows operating system while other drives are empty (At least they seem so).
The weird thing is that C:\ is not the active partition, but rather the System Reserved partition!!
Anybody knows what is going on? Any possibility?
The system reserved partition is created by windows to hold the boot manager, boot files, windows recovery environment, etc.
It's normal that your C: drive is not active. The boot manager and configuration located on the system reserved partition is what handles the booting of the OS.
The boot files, and other components, can be moved to the C: drive and reconfigured so that the system reserved partition can be eliminated. But I assume that is not why you asked.
http://www.thewindowsclub.com/system-reserved-partition-windows
