you need to install win3.11 inside dosbox as well
i don´t think that you can use an installed copy.

Oh and maybe post your system specs, mountings, dosbox version etc. somewhen.

ow does DOSBox decide whether a volume is bootable? I've got a hard disk image that boots on Bochs, but both DOSBox and virtual PC say "Missing operating system". What could be the reason?

Maybe the image is too big? Currently plain vanilla Dosbox only supports hard drive images with the size around 500MB. Or you are making a mistake in booting. As wd wrote we need to know what you are commanding Dosbox to do.

I have tried instead to run the windows 3.11 installed on my harddisk image on top of the DOSBox DOS, but Windows fails because it cannot open enough files.

Deducing from this you ARE able to at least mount the image, right? Again how did you mount it? Especially if you don't adjust path settings Windows will not run correctly (see my guide at Windows 3.1x DOSBox Guide). And I'm also not sure that running Win3.11 directly in Dosbox on an unbooted image works correctly. Try copying the whole stuff from the image to a folder on your hard drive and mount that folder as c, adjust path settings and see if that works.

i don´t think that you can use an installed copy.

Actually you can. I'm running my original Windows 3.11 installation from around 1996/97 which I backed up to CD and am able to run almost perfectly either directly in Dosbox or through a booted image. Only needed to edit some of the inis because of incompatible drivers. Of course you have to make sure all the paths are correctly set in various inis. If the Windows installation you want to use has been installed with defaults (to C:\Windows) then it should work mostly.

Thank you for all the suggestions. I did not realize that the limit of five open files is only in the disk image access routines and not a general limit of DOSBox. Maybe the documentation on this could be improved. Copying all files to a MOUNTed directory and re-installing Windows did the trick.
However, after reading the discussions on how to print to LPT1 I decided to use VirtualPC instead. I finally found working MS-DOS disks and a working disk drive and was able to SYS my original hard disk image, which works in VirtualPC without further changes.

Thank you again for your help.
Harald Fricke