Moving a document into a folder

System: Windows XP plus SP2. In a particular folder, in addition to subfolders, there may be documents by themselves. Sometimes I desire to move one into one of the subfolders. When I click on it, hold down the click, and move it to the destination subfolder, a small icon appears, a circle with a slash through it, that seems to be "No!" The document will not move. How may I accomplish this goal?