In Windows, it is possible to manage the amount of virtual memory that your system can use. By specifying the amount of virtual memory, you are actually specifying the size of the swap space on your hard drive. The swap space is used to hold pages that have changed but that need to be removed from actual memory. A page that has not changed does not need to be copied to the swap area, since the page can be loaded directly from its secondary memory location when it is needed again.
The maximum amount of virtual memory for any segment on an i386 machine is 232 = 4 Gigabytes, since it has a 32 bit address register. When a program is loaded, it has a page table created for it. In the page table are the references to all the page frames that the program will need.