Drush team recently released rc1 for version 7 of Drush, in this release they added a nice batch script to install it on windows. For this you have download latest 7.x release from https://github.com/drush-ops/drush/releases, extract zip in a partition other than OS partition (like D drive, I extracted it in D:\install\). Now start command prompt as administrator, for this search for cmd in start menu, on result list right click on cmd and choose "Run as Administrator". Now goto the extracted folder (D:\install\windrush in my case) and run setenv.bat file. It will add drush to system variables and registry. Now open new command prompt and run drush, you will see usual drush help message.