If you're looking for instructions for building GHC on Windows, they are now incorporated in the main Building Guide.

A Windows build log using Cygwin

Here is a complete, from-scratch, log of all you need to build GHC using Cygwin, kindly provided by Claus Reinke. It does not discuss alternative choices, but it gives a single path that works. Please help us to keep this up to date: if you are using newer versions, let us know whether you succeed or run into issues while following this log.

Note: starting with the  August 2008 version of 'setup.exe', adding 'http://www.haskell.org/ghc/cygwin' will not work unless you disable verification (not recommended) - until that site has a signature, you can add the dependencies from Devel->ghc-depends manually.

- Install some editor (vim, emacs, whatever)

- Install cygwin (http://www.cygwin.com)
    ; i used 1.5.16-1, installed in c:\cygwin
  - run 'setup.exe'
    Choose a Download Source:
	select 'download from internet';
    Select Root Install Directory:
	root dir: c:\cygwin; 
	install for: all users;
	default file type: unix
    Select Local Package Directory
	choose a spare temporary home
    Select Your Internet Connection
	Use IE5 settings
    Choose a Download Site
	Choose your preferred main mirror and
        Add 'http://www.haskell.org/ghc/cygwin'
    Select Packages
	In addition to 'Base' (default install), 
	select 'Devel->ghc-depends'

- Install mingw (http://www.mingw.org/)
    ; i used MinGW-3.1.0-1.exe
    ; installed in c:\mingw
  - you probably want to add GLUT 
    ; (http://www.xmission.com/~nate/glut.html)
    ; i used glut-3.7.3-mingw32.tar

- Get recent binary snapshot of ghc-6.4.1 for mingw 
    ; (http://www.haskell.org/ghc/dist/stable/dist/)
  - unpack in c:/ghc
  - add C:\ghc\ghc-6.4.1\bin to %PATH%
    (Start->Control Panel->System->Advanced->Environment Variables)

- Get and install binary release of darcs
    ; (http://zooko.com/darcs/darcsdir-cygwin-1.0.7.tar.bz2)

- In the following, shell commands are entered in cygwin bash

- Get darcs version of ghc
    ; also, subscribe to cvs-ghc@haskell.org, and possibly
    ; to cvs-libraries@haskell.org, or follow the mailing list
    ; archives, in case you checkout a version with problems
    ; http://www.haskell.org/mailman/listinfo/cvs-ghc/
    ; http://www.haskell.org/mailman/listinfo/cvs-libraries/

  - mkdir c:/ghc-build; cd c:/ghc-build
    ; (or whereever you want your darcs tree to be)
  - darcs get --partial http://darcs.haskell.org/ghc
  - cd ghc
  - chmod +x darcs-all
  - ./darcs-all get

- Build ghc, using cygwin and mingw, targetting mingw
  - export PATH=/cygdrive/c/ghc/ghc-6.4.1:$PATH
    ; for haddock, alex, happy (*)
  - export PATH=/cygdrive/c/mingw/bin:$PATH
    ; without, we pick up some cygwin tools at best!
  - cd c:/ghc-build/ghc
    ; (if you aren't there already)
  - sh boot
  - ./configure --host=i386-unknown-mingw32 --with-gcc=C:/Mingw/bin/gcc.exe --with-ld=C:/Mingw/bin/ld.exe
    ; we use cygwin, but build for windows
  - cp mk/build.mk.sample mk/build.mk
  - in mk/build.mk:
    add line:       SplitObjs = NO
	(MSYS seems slow when there are zillions of object files)
    uncomment line: BuildFlavour = perf
	(or BuildFlavour = devel, if you are doing development)
    add line:       BIN_DIST=1
  - make 2>&1 | tee make.log
    ; always useful to have a log around

- Package up binary distribution
  - make binary-dist 2>&1 | tee make-bin-dist.log
    ; always useful to have a log around
  - unpack ghc-<version>-i386-unknown-mingw32.tar.bz2 somewhere in your filesystem...

Additional notes from Neil Mitchell:

- cygwin installation doesn't quite work with the latest version because the ghc
  depends file doesn't have a .sig file with it

- for mingw installation just select the minimal package (or at least i did...)

- I got the following hiccup, which seemed transient:
----------
nmitche6@wlon1207009001 /cygdrive/c/ghc-build/ghc
$ sh boot
Booting .
/usr/bin/m4:configure.ac:1281: cannot create temporary file for diversion: Permi
ssion denied
autom4te-2.61: /usr/bin/m4 failed with exit status: 1
Booting libraries/base
Booting libraries/directory
Booting libraries/editline
Booting libraries/old-time
Booting libraries/process
Booting libraries/unix

nmitche6@wlon1207009001 /cygdrive/c/ghc-build/ghc
$ ./configure --host=i386-unknown-mingw32 --with-gcc=C:/Mingw/bin/gcc.exe --wit
h-ld=C:/Mingw/bin/ld.exe
configure: WARNING: If you wanted to set the --build type, don't use --host.
    If a cross compiler is detected then cross compile mode will be used.
mk/config.h.in doesn't exist: perhaps you haven't run 'sh boot'?

nmitche6@wlon1207009001 /cygdrive/c/ghc-build/ghc
$ sh boot
Booting .
Booting libraries/base
Booting libraries/directory
Booting libraries/editline
Booting libraries/old-time
Booting libraries/process
Booting libraries/unix
---------

- I required happy, haddock and alex to be installed

- Everything failed a few minutes in to the compile:
  http://www.haskell.org/pipermail/glasgow-haskell-users/2008-September/015429.html
  No solutions are yet known

- http://www.nabble.com/cc1-not-found-td9742088.html - looks interesting, tried adding
  c:\mingw\libexec\gcc\mingw32\3.4.5 to the $PATH

- Some hacks make the build get further:
  http://www.haskell.org/pipermail/glasgow-haskell-users/2008-September/015434.html

- Plus some things that don't work with the latest mingw:
  http://www.haskell.org/pipermail/cvs-ghc/2008-September/044945.html