In Windows 7 I went to my Control Panel and clicked on "Clock, Language, and Region." From there click on "Change keyboards and other input methods." Then, a new window will pop up with the proper "Language and Keyboards" tab selected. Click on "Change Keyboards." From here you will be able to add a language if it isn't already installed. My laptop came with only English (US) installed. Click on "Add" to find your language. Once you've done that, click on "Apply." Then, click on the "Language Bar" tab to change where the language bar shows up. This bar will help you switch from English to Spanish and from Spanish to English.
Once you've switched to a Spanish keyboard, you can write 'ñ' by clicking on the ';' key. You can also type an accented vowel by typing ´ on your Spanish keyboard (shown as '[{' on your English keyboard) plus the desired vowel.