The only thing I can think of is your paging file, otherwise known as "virtual memory". The paging file is a section your hard drive set aside for times when your physical memory (sticks of RAM) get filled up. Paging file access is much slower than RAM access. Windows is probably warning you that you have filled your paging file completely. I would look into buying more RAM. If that is not an option, at least increase your paging file size:
Right-click My Computer, choose Properties. Go to the Advanced tab and click the top-most Settings button. Go to the Advanced tab in this window and click the Change button near the bottom. Set the minimum and maximum size to some larger value (and set them to the same value). Click "Set", then OK out of all the windows. Restart.
