For convenience, I will assume that the first thing you do is:

start powershell

• Download necessary dependencies:

(new-object System.Net.WebClient).DownloadFile("http://go.googlecode.com/files/go1.2.1.windows-amd64.msi", "$HOME\go1.2.1.windows-amd64.msi")
(new-object System.Net.WebClient).DownloadFile("http://bitbucket.org/tortoisehg/files/downloads/tortoisehg-2.11.2-hg-2.9.2-x64.msi", "$HOME\tortoisehg-2.11.2-hg-2.9.2-x64.msi")
(new-object System.Net.WebClient).DownloadFile("https://msysgit.googlecode.com/files/Git-1.9.0-preview20140217.exe", "$HOME\Git-1.9.0-preview20140217.exe")
(new-object System.Net.WebClient).DownloadFile("https://launchpad.net/bzr/2.5/2.5.1/+download/bzr-2.5.1-1-setup.exe", "$HOME\bzr-2.5.1-1-setup.exe")

• Install dependencies

msiexec /i $HOME\go1.2.1.windows-amd64.msi /quiet
msiexec /i $HOME\tortoisehg-2.11.2-hg-2.9.2-x64.msi /quiet
$HOME\bzr-2.5.1-1-setup.exe /SILENT
$HOME\Git-1.9.0-preview20140217.exe /SILENT

• Add Apropriate environment variables

setx PATH "$env:PATH;${env:ProgramFiles(x86)}\Git\cmd;${env:ProgramFiles(x86)}\Bazaar;${env:ProgramFiles}\TortoiseHg"
setx GOPATH "$HOME\golang"

• set variables for current session as well:

$env:PATH = "$env:PATH;${env:ProgramFiles(x86)}\Git\cmd;${env:ProgramFiles(x86)}\Bazaar;${env:ProgramFiles}\TortoiseHg"
$env:GOPATH = "$HOME\golang"
mkdir $env:GOPATH

• Get juju-core and its dependencies

go get launchpad.net/juju-core/...

• Get our patched version from git. This contains the windows support:

rm -Recurse -Force $env:GOPATH\src\launchpad.net\juju-core
git clone git@github.com:cloudbase/juju-core.git $env:GOPATH\src\launchpad.net\juju-core
cd $env:GOPATH\src\launchpad.net\juju-core && git checkout rebase-1.19

• Because of the way Go works, you will always get the latest revisions of dependencies. This might sometimes break the build of juju. The checkout from 19 April needs the following hack:

cd $env:GOPATH\src\code.google.com\p\go.crypto\ssh
hg update -r 191:2990fc550b9f # hack for this particular checkout of juju

• Build juju-core on windows

go install -v launchpad.net/juju-core/...

• Binaries can be found in $env:GOPATH\bin

IMPORTANT NOTE DRAGONS AHEAD

If you ever get a bunch of errors like the following:

conflicting definitions for @

Simply remove $env:GOPATH\pkg\windows_amd64 and try again.