can anyone help a newb novice?

hi...i have a small problem which you may think even a rhesus monkey could solve but im new 2 all this so any help will b appreciated...i will explain...my current pc got damaged resulting in the mainboard getting cracked and is beyond repair
it has an 80gig hd which i presume will b fine...
ive been given a pc which is a miserly (dont laugh)
10 gig and both run on xp...what id like to do is put the 80 gig into the other one but dont know how to as ive never really "looked under the hood"
so to speak, thanx if you can help!!!!