Yes, Keylight shipped with After Effects CS3. Unless you have the downloaded trial version, it installs as part of the standard installation. Make you you opted to install the optional 3rd party components. Look in Effects under the key category. It should be there.
The Foundry issued a maintenance release for the version of Keylight that shipped with AE CS3 (1.2v5) that brought it up to version 1.2v8. Check with The Foundry
http://thefoundry.co.uk/pkg_plugins.aspx?ui=A5FB8E97-D752-4E01-AC7D-17B39B7E1192. You'll need the activation key of the original keylight plugin to activate the 1.2v8 plugin.