Depending on your version of Windows you might be able to download official language packs from Microsoft or windows update. If not you can use a unofficial unsupported third party software like Vistalizator and a MUI for your language(remeber to check your OS's bit).
Alternate display languages are only officially supported in Windows 7 Ultimate and Windows 7 Enterprise; to change the language for these operating systems you can use Control Panel>Region and Language>[Keyboards and Languages] tab to install and uninstall display languages. Alternatively, if you do not own Windows 7 Ultimate or Enterprise you can use Vistalizator to change the language of your system.
If your computer is running on Windows Vista : >Control Panel -Clock, Language, and Region -Change keyboard or other input methods -Change keyboards -Add.. -Spanish Then you can toggle from each keyboard using Shift+Alt. Other operating systems should be around the same. Look for language/keyboard settings in the control panel.