Thank you so much Apprentice ! The win 98 compatibility fix worked fine, although oddly it kept reverting to the desktop picture all through loading, and only came back once I clicked the taskbar tab. Once the game started it was fine. I only play single player missions, so that fix was good enough for me. Much appreciated !

Thanks for your help with DCX.9 Apprentice.
Its loading and working perfectly now.
Perhaps you could help me with another problem too ?
I have downloaded the Secret Weapons of WWII expansion, and when I browse for, and select the file path to the folder,
in my case ...  C:\Program Files (x86)\Battlefield 1942\Battlefield 1942  ... the installer says, Cannot continue setup No Battlefield!  
What am I doing wrong ?

your game path is very weird i just checkt 3 bf1942 installs and  all are installed in

C:\Program Files (x86)\EA GAMES\Battlefield 1942

or

C:\Program Files (x86)\Origin Games\Battlefield 1942

have you just copy paste folders   ?

Yes, each time I have to get a new laptop I copy all my games from one to another. Some work, some don't.
If I changed the name of the games folder now, would the game stop working ?
I do have an EA GAMES folder with Need For Speed etc in it. Can I move it to that folder ok ?

problem is you just copy paste your games so now  a lot of registry keys are now missing

to fix this you first need to reinstall bf1942 so your folders are fix in regedit

to save your mods

first reinstall bf1942 in the ea games folder after just move the mods folder in

C:\Program Files (x86)\Battlefield 1942\Battlefield 1942  

to

C:\Program Files (x86)\EA GAMES\Battlefield 1942

after all this im sure the Secret Weapons of WWII expansion installer will work

Thanks Apprentice. I ended up ditching all my old copies and downloaded the HD Anthology.
Now I have everything in the right folder and all mods and extensions working properly.
What's more, I now have full screen options, and British and German voices instead of American for everything.