Install Windows Powershell

Windows 7 & Windows Server 2008 come with powershell 2.0 installed by default

Download Powershell 3.0 for Windows 7, Windows Server 2008 & 2012

for all others you can download the installer here

Run Powershell

Start > Run > Powershell

Set-ExecutionPolicy remotesigned -force
cd ([environment]::GetFolderPath([environment+SpecialFolder]::MyDocuments))
git clone git://github.com/ilude/WindowsPowerShell.git
exit

restart powershell

And run the following command to setup git configuration info and aliases

Setup-Git