if diablo is already installed then how can i install it again???

plzz help me out

uhh.. you dont want to?

just copy the diablo 2 folder and give it a different name like Diablo II (2) and theres your second copy.

ok thx but if i have 2 installed now how do i make them use different cd keys?

You don't have to copy anything, there's a special program for switching cdkeys.
A guide on how to do that can be found here:
http://www.diabloworld.com/forums/showthread.php?t=62341