You have processes running in the background all the time and these use system ram. To see what proceses are running hold down CTRL-ALT-DELETE and this will bring up the task manager. Click the processes tab and you can see what's running. I would reccomend NOT messing with anything in here as it can ruin your machine. 64mb dosent like a great deal of useage anyway.
Life is too important to be taken seriously - Oscar Wilde