I am using Windows 7 as well and had this problem too.
After I took the steps outlined in Scott Marchant’s answer, bower worked.
Find the location of your npm global binaries: npm config get prefix. This path may look something like C:\Users\username\AppData\Roaming\npm.
Add the path from step 1 to your Path.Open the Windows Control Panel, search for environment, then click on either edit environment variables for your account, or Edit the system environment variables`.
Find the variable named Path or PATH, or create one if it doesn’t exist.
Paste the path from step 1 here (; delimited).
You may need to restart your command prompt window.
You should now be able to enter bower commands.