Did you build SDL yourself? Also did you build SDL_net?
If yes to both, it's probably a good idea to rebuild SDL_net against your new SDL; also, when I built SDL 1.2.15 the files ended up in different places, namely SDLmain had its own subfolder appropriately named "SDLmain" and the library file was compiled under there, so you may want to check that.

No, I didn't build SDL; I did exactly what I did with 1.2.4, which was to download the Visual C "development libraries" from this page:

http://www.libsdl.org/download-1.2.php

As I said, the same thing works perfectly when I have 1.2.4 in the additional directories, not 1.2.5.

Visual C++ says it can't open the file, which makes me wonder whether it can find the file. But the "command line" option in Visual C++ seems have a correct libpath= parameter, pointing to 1.2.5, just as the properly-working setup has a command line pointing to 1.2.4.

Ahh I see now...you did change the include path to the new one too, right?

Ah - you're right: in the Property Pages, under Configuration Properties, C/C++/General, in the "Additional Include Directories" I forgot to change

../SDL-1.2.14/include

to

../SDL-1.2.15/include

But now that I've done that, and cleaned the solution, I've given myself another problem. The build output gives a fatal error:

d:\development\dosbox with char9\include\timer.h(23): fatal error C1083: Cannot open include file: 'SDL.h': No such file or directory

Of course SDL.h is in ../SDL-1.2.15/include.

I'm sorry to come back to you with these beginner questions, but if you can guess something else I need to do, I'll be grateful. Obviously, I got this right once, and am only hoping to figure out how to get it right again.

OK - PLEASE IGNORE EVERYTHING I WROTE EARLIER - and many apologies for wasting time and bandwidth!

It was a stupid beginner's mistake - I put the SDL 1.2.15 directory one level above where it should have been. I saw an SDL 1.2.14 directory and put the 1.2.15 directory next to it - when I should have looked one level down, where I would have found another 1.2.14 directory and should have put it there.

Heartfelt and deep apologies again for wasting other people's time and the board's bandwidth. This really was something I should have figured out for myself instead getting other people to explain to me that 2+2 does not really equal 22.