How can i clone my hard disk

My current system is operating on windows vista business, AMD 64 & and hdd size of 160GB. I have another hdd of the same size but with operating systems loaded on it but i no longer use it. I want a complete replication of the my current system on the idle disk. How can i do this?
do i have to format the un-used one to remove the OSs on it?
What i actually want is an exact replication of the current system to the idle drive.

Please help me out with steps to take.
TIA