160GB loss some GB because XP without SP1 (or SP2) has a up limit 137GB. With SP1/SP2 installed, you do not loss any GB.
No loss GB with 120GB.
When you buy a HDD with 120gb, you only get 120/1.018/1.028/1.028=110.5GB.
because 1G = 1.028g, 1M = 1.028m, 1K = 1.028k. More over 1B=8b.
Marketing uses g, m and k and IT uses G, M and K.