If you are saying you have two physical hard drives; one with Win 7 installed and the other hard drive is empty (but with two partitions), then it is pretty simple.
All you have to do is install both hard drives in the PC. Connect the power cord and assuming both are recent hard drive they should have a SATA port. Therefore connect the hard drives to the motherboard using SATA cables. The motherboard's manual should tell you where the SATA ports are if you are unsure.
If you have older hard drives using the EIDE port and your motherboard does not have such a port, then you need to buy a EIDE adaptor card (PCI or PCI-e 1x slot depending on the motherboard)..