Right-click on the My Computer icon on your desktop and select Properties. Click on the "advanced" tab, and then click on the "Environment Variables" button.
In the bottom list box (labeled "System Variables"), scroll down to the "Path" variable. The value should begin with "c:\ruby\bin;" (if you installed to the default location). If it doesn't, then something went wrong during the installation (perhaps you didn't have Administrator rights).
Anyway, if its not there, you can double-click on "Path" to edit the value and add it (you may need to reboot for the change to take effect).