configure seperate hdd

hi there i have just installed a second hdd i have a 20gig running myos xp pro and 120 gig which is fornatted and empty wjhat it is. i want to just use the 20 as my running hdd and use the other for all othere stuff. but when im trying to install programs in just trys installing onto my 20gig hdd. i dont want this i just wanrt that to sit there doing nothing apart from running my syste. how do i use my 120gig hdd for this. sorry ifn this is a thick question but im a nobe thanks to all for there view on this