If you want 32-bit Windows with 3.1-appareance, you should look for 
Windows NT 3.51.
>Does anyone know which files produce the window frames and the maximise and
>minimise buttons on the title bars in Windows 3.1 - the things that make
>Windows 3.1 look like Windows 3.1 ?
>What sets the font in the dialog boxes, and minimises programs to a proper
>32x32 icon, rather than a Windows 95 bar?
>Are these instructions somewhere inside \windows\system\user.exe, for
>example?
Hi Stephen,
Too many questions in single message! :-) Well, I'll try to
answer some...
The visual aspects of Win3.1x interface are controlled by many
files, not single one.
The control buttons are stored as bitmaps within graphic card's
driver, fonts and dialog style are handled mostly by COMMDLG.DLL,
CTL3D.DLL and similar libraries, and also affected by USER.EXE.
Concerning the minimized icons - I have no exact information, but
IMHO it's a PROGMAN.EXE business.
You can search the web for many Win3 "interface enhancers" - many
of them include good documentation about related things. For
example, I'm currently using the next add-ons:
'Makeover for Windows' by Playroom Software (including OSFrame,
ButtonMaker and IconMaker),
'Kframe' by Kauru Nakajima,
'More Control' by Sloop Software,
'All3D' by Andreas Furrer,
'Ministry of Buttons' by Jonathan Mote,
'Instant File Access' by Alexoft Software,
COMMDLG.DLL patch by Maxim Gready,
USER.EXE from Win98 MINI.CAB archive,
and many other patches, modifications, improvements and hacks -
I've just figured out, that I'm unable to give a full list of
them!
>What would happen if you went into an installation of Windows 95 and
>replaced the Windows 95 versions of the files containing these windowing
>instructions (like user.exe maybe) with the Windows 3.1 versions?
Nothing good, I suppose! ;-)
>Is it possible to make Windows 95 boot up looking like Windows 3.1 ?
There's an option in the Win95 installation, where you can select
which interface do you want to use - Win95 or Win3.1 one. If the
latter is selected, the Win95 looks pretty much similar to the
Win3.1x, although it isn't 100%-identical.
WBR,
Artur