This could be to do with your virtual memory settings. Virtula memory takes up space on your hard drive so that files can be quickly accessed instead of being 'fetched' form the storage. GO to control panel then system then advanced and then performance settings it should tell yout here somewhere how much virtual memory is being used of the hard disk, don't change it unless you knwo what your doing, if it isn't high or the amount of memory youve lost then im not sure right now what it is