Windows 7 100% Memory Usage (without any process listed as using that much memory)

You might want to check in the Resource Monitor as well. It shows much more detailed information about where memory is being used. You can access it from the start menu, or by clicking "resource monitor" in the Task Manager's performance tab.

Check eventually with Process Explorer from Sysinternals, to have more details about all processes, and which one is taking memory.

Keep in mind that the low CPU is not surprising, it doesn't need to use a lot of it, to use simply memory, and a lot of data.

Make sure you click "Show processes from all users" in task manager. Otherwise it hides the system processes. Switching to 7 from XP this tripped me up a few times troubleshooting CPU hogs; I kept forgetting to click the button.

(In XP it was a persistent checkbox; hope I can find a way to make it persistent in 7...)