A. Virtual Memory makes up for the lack of RAM in computers by using space on the hard disk as memory, Virtual Memory. When the actual RAM fills up (actually its before the RAM fills) then virtual memory is created on the hard disk. When physical memory runs out, the Virtual Memory Manager chooses sections of memory that have not been recently used and are of low priority and writes them to the swap file. This process is hidden from applications, and applications views both virtual and actual memory as the same.
Each application that runs under Windows NT is given its own virtual address space of 4GB (2GB for the application, 2GB for the operating system).
The problem with Virtual Memory is that as it writes and reads to the hard disk, this is much slower than actual RAM. This is why if an NT system does not have enough memory it will run very slowly.