In the end it was something else.
No idea what to call this problem, really. I use an irc client (mIRC) that uses CTL+K to implement color codes to colorize the text. At some point the normal "black box" symbol that normally comes up when you use this key combination has changed to what appears to be a small "L" symbol (not the letter "L"). I thought perhaps I had inadvertently changed a setting in the client program, but I have tried downloading a fresh copy of the program and using other versions of the same program, and the same thing still occurs, therefore I am thinking perhaps I have somehow managed to change some setting in WinXP. I cannot find anything on the internet about how such a setting would change.
I am using Windows XP, updated to current. The new symbol does not affect the operation of the irc client program, nor, apparently, anything else in the normal operations of my system.