The .SWP file is more commonly known as the "swap" file, and is used by the system to "swap" out memory to the disk, temporarily. Windows uses this as a way of increasing the amount of memory available on the system, a method called virtual memory. Unless you turn off virtual memory, this file won't go away; in fact, having virtual memory turned on is a good thing. In short, this file is used by Windows for normal operations and should not be deleted.
This was first published in February 2002