No new info here, but I thought some details should be clarified. As mentioned already, if you look inside your PC there's a strip of wires which plug into your existing CD/hard drive. There should be a another connection halfway along it labelled "slave". Plug this into your 2nd HD, manually set this HD as slave by amending the "jumpers" on the back (as anthomson says) and you will NOT need any operating system on the 2nd HD.
Then follow lard's instructions on disk management (also in control panel under "administrative tools").
Actually, you may want to put/leave an operating system on this 2nd HD just in case the main one fails altogether, then you can reverse their roles, use the new one as slave and retrieve files from it in any future emergency.