When Windows runs out of "hard" memory it will transfer unused data to the "page file" to free up hard memory for the current process. The amount of space in the page file can be set by you. What you are describing is Windows is running out of page file space and will automatically allocate more disk space to the page file.

Search for "page file" in the Windows help section. You will find instructions on increasing its size.

and life goes on...

Jack