Following steps installed drush in my windows 7 PC's & laptop seamlessly. Please ignore 1st two steps if you already have a web-server-stack running in your machine.
1. Install VC11
https://download.microsoft.com/download/1/6/B/16B06F60-3B20-4FF2-B699-5E...
2. Install XAMPP-5.6-VC11
https://www.apachefriends.org/xampp-files/5.6.32/xampp-win32-5.6.32-0-VC...
3. Install GIT
http://git-scm.com/download/win
4. Install composer
https://getcomposer.org/Composer-Setup.exe
5. Install drush using compser
in git-bash type: composer global require drush/drush
6. in bash - navigate to sites folder
7. check environment by typing following commands in bash one by one
php --version
mysql --version
composer --version
drush --version
Incase any of the above commands returns error, make sure to update environment variables accordingly.
Makesure that your environment variables have these entries (depending upon your install location & user name)
C:\Users\Admin\AppData\Roaming\Composer\vendor\bin;
C:\Users\Admin\AppData\Roaming\Composer\vendor\drush\drush\;
C:\Program Files\Git\cmd;C:\ProgramData\ComposerSetup\bin;
C:\xampp\mysql\bin;
C:\xampp\php;
8. Finally in sites\defalut\settings.php change the host from localhost to 127.0.0.1
Hope this helps