Besides spyware, adware, and unnecessary services that stays resident in memory, you probably have several (or more than several) legitimate programs that keep "stubs" in memory. You'll generally see icons for those in the System Tray (or System Notification Area) - the bottom right-hand area of your taskbar, or in the Startup folder. Each one of those only takes a little bit of memory, but they add up. Sometimes you won't see them in the tray or the startup folder, but they'll be loaded anyway. These stubs are supposed make access to the program faster, but I personally don't see that much of a difference, except that they will bog a machine down. Some of the common programs include anything by Adobe, Quicktime, Realplayer, AIM or any other instant messenger, Microsoft Office, and display control panels. Obviously, that's not a complete list, but I've seen some trays that take up half of the taskbar area.
By comparison, I have only 22 services, which includes AVG anti-virus, running in the background.