The OS and applications installed will cause it to vary. If this is Windows 8, and your other system are Windows 7, that would do it alone. Windows 8 uses more RAM than Windows 7. Even a handful of apps that are open, or a few apps that run in the background on this system but aren't on the other systems would also do it. If you have 4 GB of RAM, seeing 2/3 of it in use when you aren't really doing much isn't uncommon with Windows 8.