i am using windows proffessional 64 bit now and it will expire in 2 months . i want to buy a 64 bit os but im confussed about this upgrade business . whats the difference ? thanks johnny

The upgrade requires you to have windows xp. When you try to install Vista it will validate your copy of XP, then start installing Vista. The downside is that after Visat is installed your XP key is no longer valid, so you cant use it on another machine. If you get the full retail version you dont have this problem, as it requires no OS to be installed first.