In the control panel (start --> control panel), open the Region and language tab. In Vista it will be Clock, Language and Region.
Step2
On the language tab, click on details. In vista, click on change keyboards or other input methods.
Step3
Click on the language that you would like and save it as an added keyboard. In vista, click change keyboards and then you can add a keyboard and save it.
Tips & Warnings
You must select one of your keyboards as a default. Choose the one that you will use most often