So, I notice this version of Descent is included in the game files. But I started DOS Box and then clicked on the executable and the game starts, that is I can only get as far as the main menu, I cannot start a new game in any way. Can anyone hep? I tried rebirth but must not have configured it correctly as I couldn't get that game started at all.

You initial posted makes it sound like you might have tried to run Rebirth or D2X-XL from within DOSBox. You wouldn't want to do this since these recompilations of the game engine are designed to run natively in Windows.
As far as getting D2X-XL to work, if I remember correctly, it is actually more complicated to get the original Descent working with it since it also requires Descent 2 resources, but in general, the installation procedure is the same: just slap the appropriate files in the directory of the extracted Rebirth or XL package and you're ready to go.
For your reference, here is my Descent Rebirth (0.54) directory listing:
apeman.plr
apeman.plx
apeman.sg0
CHANGELOG.txt
COPYING.txt
d1x-rebirth-gl.exe
d1x-rebirth.ico
d1x.ini
d1xrdata.zip
demos
descent.cfg
descent.hi
DESCENT.HOG
DESCENT.PIG
gamelog.txt
jukebox
libogg-0.dll
libphysfs-1-0-1.dll
libvorbis-0.dll
missions
README-D1X.txt
screenshots
SDL.dll
SDL_mixer.dll
stdout.txt
The only thing to note here is the that the Chaos.hog and chaos.msn files should be placed in the missions directory to give you access to the multiplayer maps. Hope that helps!

Thanks Dr. Colossus, That did the trick. I am playing in beautuful hi-res.

Glad to be of assistance :)

If you do not just want to play in hires, but with colored light, high res textures and models, smoke fx, and and and ... get D2X-XL.
For support, register on the D2X-XL forum.