Virtual Memory

The most simple and comprehensive definition of virtual memory can be found on wikipedia, it states that Virtual memory is a computer system technique which gives an application program the impression that it has contiguous working memory, while in fact it may be physically fragmented and may even overflow on to disk storage. Systems that use this technique make programming of large applications easier and use real physical memory (e.g. RAM) more efficiently than those without virtual memory.
http://en.wikipedia.org/wiki/Virtual_memory
Information: Virtual Memory is always in use, even when the memory required by all running processes does not exceed the amount of RAM installed on the system.
How do I know the recommended virtual memory size? It’s easy, its 150 percent of the available RAM size…
http://support.microsoft.com/kb/555223

Maximum size for virtual memory on 32-bit OS is 4GB, and in 64-bit OS is 16TB.
http://support.microsoft.com/kb/294418