Assuming you are running windows 7, you need to add path to PHP.exe to windows environment variable :
Follow following steps:
From the desktop, right-click My Computer and click Properties.
In the System Properties window, click on the Advanced tab.
In the Advanced section, click the Environment Variables button.
And, in the Environment Variables window , highlight the Path variable in the Systems Variable section and click the Edit button.
Add or modify the path lines with the paths you wish the computer to access.
Add path to your php installation after semicolon like
e:/php/php.exe