Although virus scanners are a necessary evil, resident virus scanners are not. Through some perhaps misguided sense of control of my PC I prefer to manually scan suspicious files as the need arises, keeping memory and CPU time available for more interesting purposes.
Being the frugal person that I am, I rely on the free and sufficient AVG Anti-Virus. It has served me well, but has had one small annoyance. Even though it is not performing its resident virus scanning function, it places an AVG icon in the system tray. Although this icon can be clicked, and the AVG Control Centre shut down, it always returns at startup.
When I reinstalled Windows a few weeks ago I took special care when installing AVG to include only the minimal components necessary. The result of this was that the icon no longer appears automatically. It turns up when I manually run the AVG Control Centre, but when I shut it down it remains closed.
Although it's a bit annoying for a product to not include a feature such as the ability to shut itself down once and for all after installation, I accept these little eccentricities as the price of using free software.