Exact commands used to mount drives, and start the game please.
A listing of what is in your top-most directory on the DOSBox C-drive would be nice too.

mount c c:\SCREAMER
mount d E:\ -t cdrom (I've tried the other two aswell)

Dos box opens happily, and then I try and run screamer, at which point t comes up with the exact words "Cannot open main file C:\SCREAMER\data\GEN.dat"

I've checked and the file is there. As said before the game runs under VDMS with Sapucdex, but crashes at the same point on Lindberg and Lake Valley (corrected from mountain pass) and occasionally on the first map. I've tried most things, and it still crashes

Apologies for the double post, but just to say that it also has DOS32A thingy done to it. The rig is a Laptop, with a 1.5 GHz Pentium centrino, 512mb of RAM, 64MB ATi Radeon 9700, 40GB hdd.

Basic mistake: mixing mounted drive paths with real drive paths.

Try this:
create a C:\GAMES directory (there you should put all dos games to),
then copy the whole C:\SCREAMER directory into C:\GAMES so you
have a directory C:\GAMES\SCREAMER
Now in dosbox, use "mount c c:\games"
when you change (in dosbox) to c: and type dir, you'll see a directory
"SCREAMER". Compare this to your old mounting. Now go into the
directory SCREAMER (still in dosbox) and start the game.

i agree with WDs posting, but you also should check your sreamer.cfg in the main directory, if there is some mismatch with your settings.
btw i know your problems with VDMS. the game crashes for me on every track with hi resolution.

if you get screamer running with dosbox try the dynamic core and increase the cycle settings above 40k, but with a 1.5 GHz Pentium Centrino you would get no smooth gameplay at all.

Now it can't find the cd.

Did you mount the cdrom with -t cdrom? Maybe you got to
specify the cd label additional to that (see the readme for
how to do that).

Open and edit screamer.cfg in your screamer directory
By example: I have screamer in root folder of unit D:

My screamer.cfg is :

d:\ d: d:\ d

This is for running installed Screamer with not cdrom from drive D

Screamer found DATA\GEN.DAT 😁

This is for a Screamer installed in Screamer folder in drive C

c:\SCREAMER c: c:\SCREAMER c

Sorry english is not my first language