Are you referencing video game logic (such as in the recent game Thief) where guards wander aimlessly? Or, perhaps, you are referencing famous "standing guards" such as those at Buckingham Palace. In either case, the answer would basically be no. While specific areas such as gates would absolutely have been "guarded," the concept of a guard is a bit of a misnomer. In Medieval England (the sort of time and place I assume you are referencing), local and regional lords would have military forces proportionate to their own power. "Guards" as seen in many media representations did not exist, per say. Instead, those military forces would be expected to guard their lord or lady's property through the threat of violence and, in the case of the local keep itself, their physical presence. Keeping large numbers of armed men around to do nothing was extremely expensive. Most armies and/or combat forces were raised only when needed. Therefore, professional fighting men were a bit too expensive to have guarding furniture.

Essentially, movies and video games considered medieval guards to be a "police force" of sorts. In reality, such a police force did not exist in the way that we know it today. Yes, as time passed and cities became larger, some cities would have a city guard of sorts that did handle some policing activities. Also, monarchs and important figures such as Cardinals and Dukes might have personal fighting forces used to guard their person. Once again, though, these forces would be considered part of what we would call the military, not the a police force.