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That actually isn't unusual. Even when the computer appears to be sitting there to us there is a lot going on in the background. If you "Ctrl + Alt + Del" and go to the "Processes" Tab you can all of the background action that is happening.
If you'd like your computer to really slow down while you are away you'll just need to adjust the power settings. Go to Start-> Control Panel -> Power Options.
Under the "Power Schemes" Tab you can select to turn off your hard drive after a certain length of time. This will get it to stop while you are away! However, when you return to your computer it will take longer for it to fire back up. It is a trade off.
I hope this helps! If so, please don't forget to "Accept!"