A: Yes, you can, and it's usually fairly easy, at least with modern operating systems and browsers.
For the setup you have, which is one of the most common combinations, all you have to do is click on the Tools menu at the top of Internet Explorer, then select the Internet Options option. (Depending on your version of Windows, you can also get to Internet Options from the Control Panel.) Then pick the General tab, then Languages, then (if you have Internet Explorer 7) Language Preferences.
Chances are that once you reach the place where you can select your browser languages, the only language listed there is English. To make certain sites appear in Spanish, press the "Add..." button to see a list of languages. Select Spanish [es], and then you'll see both English and Spanish listed as language options. You can change the order of the languages; if you put Spanish above English, then Web sites that are available in both English and Spanish will display in Spanish. If they aren't available in Spanish, they will display in English (or whatever language they're written in) as usual.
When you see the list of languages, you'll also see Spanish variations such as Mexican Spanish and Argentine Spanish. If you prefer some variety of Spanish, you can additionally select it and add it to the list, then place it on top. This will make a difference very rarely, as very, very few sites have multiple Spanish versions.
Making this change does not affect the language used in the browser menus, only the language in which pages display.
The procedures are similar for the other common browsers, including those on non-Windows platforms:
- In Opera 9.5x, select Tools, then Preferences, then the General tab. You can select languages from that tab by pressing the Details button.
- In Firefox 3.x, select Edit, then Preferences, then Content to get to the languages selector.
- In Safari on Mac OS X, the language selection is made in the configuration of the operating system. Select System Preferences, then International, then Language.
To see if your change in language preferences has worked, simply go to a site that offers content in multiple languages based on browser settings. A popular one is Google. If your changes worked, the Google home page (and all search results) should appear in Spanish.
Note that this change works only with sites that recognize your browser configuration and act accordingly. For other multilanguage sites, which usually display in English or the main language of the home country by default, you'll have to pick the Spanish-language version from the menus on the site.

