Ralf has helped me resolve the issue, and I thought I'd post the solution here in case someone else is affected.

It appears that there is something wrong with my Windows cache, or with how the installer and update are interacting with it. Whatever the issue is, it had the following effects:

Running the installer in repair mode triggered a "Your license key is invalid" message. Running Update 28 took care of that issue (though it did prompt me to enter my serial number, which it doesn't normally do), but removed the patch.ini file from the system folder and the cfgmenuconfig.exe file from the data folder. It may or may not have removed other files as well - I have no idea.

Ralf sent me an update program that skipped the Windows cache steps, and that updated me successfully and without removing those two files.

So, I'm up and running, and hoping that if the issue is with my Windows cache this is the only thing that's affected.

I'd like to claim this one. I probably won't be able to get it done before the deadline (though I'll try), but I've got an idea for it I'd like to develop anyway.

Yep, it's there in the System subdirectory. As far as I can tell it looks like it should… I'm no programmer, but there are lots of lines to add various .mnu files and their associated bitmaps (which I assume are the buttons?).

Pappy is what my kids and I have decided my grandchildren will call me (when I actually have grandchildren, which is still a few years down the road, I hope).

Nice, Ricko! I love your other style, but this is awesome too!