App Store - Updates Available window freezes


I'm fairly new to Mac.

I have a mid-2011 21.5" iMAc with OSX 10.8.2. Every time I have an update available from the App Store, a little window pops up over all my windows in the top right-hand corner. It reads, "Updates Available New Software is ready to be installed" and has a Details and Update button.. After I update the apps, this window never goes away. When I mouse over it, I get the pinwheel icon. I can't close the window, and App Store isn't even the list of apps to Force close. To get rid of this annoying window, I have to reboot my machine.

Is this just my machine, or is this a bug in OSX? Is there any fix?