Is there a keyboard shortcut or command that brings this up?

I don't have this running on startup. I don't start it up intentionally. But occassionally this just appears in my tray. I have no idea If I'm hitting some kind of shortcut key while doing something else that brings this up.

You can control whether this icon shows in your "tray".  If it's appearing only once in awhile, the setting may be "Notify me when this connection has limited or no connectivity"; it may be hidden in your tray (click on the < to the left of the tray to unhide items in the tray); you surely would notice it if it was there ht-click
Chances are you are experiencing a change in connection speed, like from 100Mbps to 10Mbps, possibly caused by an intermittent change in the ability of your Ethernet cabling to carry a high-speed data rate.  When you hold the mouse cursor over the tray icon, a box will appear that tells you your current connection speed, or that "A network cable is unplugged".  You might not perceive any change in connection speed if you are just surfing the Web, but Windows can monitor any changes and report them to you.  If you don't want to see this, change the settings mentioned in the first paragraph of this post.

If your connection speed is cycling between 10 & 100 Mbps and you want it to stop doing this, try changing your Ethernet cables out for new Cat 5e cables.

Hope this helps...

thanks - that may explain it.

There was a blown fuse on the circuit that has my router &  network hub on it. The router is on battery backup but the hub isn't, so XP may have seen this as a problem with the network connection.