I encountered this issue too. I'm using Git Bash, hence the Unix-style $ prompt on Windows.
$ rails db
Couldn't find database client: psql, psql.exe. Check your $PATH and try again.
Here's what I did:
In Windows 7, navigate to:
Control Panel
All Control Panel Items
System
Advanced System Settings
Environment Variables
from the System Variables box select "PATH"
Edit...
Then append this string to the existing PATH Variable Value:
;C:\Program Files\PostgreSQL\9.2\bin
and click "OK" three times to exit the menus.
Now, close the console and restart it.
Navigate back to the directory of your Rails app. In my case, this is accomplished with:
$ cd rails_projects/sample_app
Then, try again:
$ rails db
sources:
How do I put PostgreSQL /bin directory on my path in Windows?
http://railscasts.com/episodes/342-migrating-to-postgresql?view=asciicast