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