Windows Explorer Address Bar Disappearing

I'm running Windows 7 Home Premium 64bit, + SP1 and fully patched. 6 GB of RAM and over 1 TB of free space on the designated scratch drive. No 3rd-party modifications or hacks to the OS or GUI. Sporadically, my Windows Explorer folders lose their Address Bar. Sometimes this happens during a session, sometimes when I've rebooted the PC after previously shutting down and leaving some Windows open (but no apps running), but I've never actually see the Bar go.

What remains of the window will still be functional - I can navigate to other folders and files with the Tree pane on the left, for instance - but there'll simply be no Address Bar. (Which means no search input, refresh button or back and forward buttons.) I can toggle the Menu Bar on and off, but it doesn't bring the Address Bar back. Usually I simply end up closing that window; when I open a new one, it has an Address Bar, no worries.

It's not all Explorer windows that lose the Bar at the same time; typically, it will only be one out of maybe three or four I have open. And I haven't been able to find any pattern to when the Address Bar goes; it doesn't happen every time I use my PC, and I haven't spotted any pattern to what app(s) I might be running when it happens. I've been trying to spot a pattern, but haven't yet.

There may not be one; if I have two Explorer windows open and navigated to the same folder on the same drive, only one of the two might lose its Address Bar, so it doesn't seem to be location-specific. It's a completely legit, licensed retail copy of Windows, and I'm not running any pirate or hacked software. I have Nortons Internet Security 2011 installed and keep it maintained. And I don't seem to have any other problems - no identified misbehaviours, no error messages.