On a PC you don't generally have access to all your RAM. You might have 4GB in your machine, but only be able to access and use 3.5GB. This is not a Windows problem specifically. Rather, it has more to do with motherboards and BIOS implementations. So that's Windows machines. But how about your Mac Pro? You probably have a pretty good idea how much RAM it has inside, but how many of those installed gigabytes can you actually use? Cheers!