Software Update not updating

I am running Snow Leopard. A few times a week, the classic "You have software updates" window pops up on my screen. However, it does not allow you to press any of the buttons, either continue, cancel, or view. Whats more, it won't let you quit, and I have to use the force quit menu to get rid of it. You do not need to restart to complete the update. The bug has been happening for a few weeks, and it is kind of annoying not being able to update. Any ideas?