As the others have mentioned, it sounds like your virtual memory.
You have two types of memory: The first is your primary physical memory, also known as RAM. The second is your secondary virtual memory, also known as a swap file. Windows automatically sets up a swap file on your drive and normally makes it 1.5x the amount of RAM you have. However, it also makes variable sizes so that it could be anywhere from 1.5-3x the size of your actual physical memory.
You will need to go to your system properties and set your virtual memory to a static size that will not fluctuate. I would post instructions on this, but I haven't used ME in years to remember exactly where it is - just right click "My Computer" and click on "Properties" and then access something on the lines of the "Advanced" tab. There should be a few variables you can screw around with. DO NOT CHANGE ANYTHING IF YOU DON'T KNOW WHAT YOU ARE DOING. ALSO DO NOT DISABLE OR LESSEN THE SIZE OF YOUR SWAP FILE. Doing so could cause your system to become unstable; removing your swap file will probably make your computer run slower because it will be forced to use only your primary memory - and unless you have a ****load of RAM, you don't want to do that.
All you want to do is set the minimum and maximum to the same size.