If your typing comes out in one language but you want it to display in another, LangOver can make the switch easy for you. LangOver (free/donationware) isn't translation software, and won't allow you to select existing online text in another language and convert it to your own language. It instead works with the languages you have already enabled in Windows, and allows you to quickly switch snippets of text you type between those languages.
To see which languages you currently have enabled in Windows XP, open the control panel in category display mode and select the Date, Time, Language and Regional Options category. Then choose the Add other languages task, click the Languages tab, and click the Details button. You can then add additional languages by clicking on the Add button.
After that, or if you already have additional languages enabled, you can hit F10 to switch a line of text from a selected language to English or vice versa. Doing so will also switch the keyboard layout so that additional text you type is in the alternate language. Hitting F10 switches back.
LangOver requires Microsoft .NET 2.0, and its developer requests donations. The tool would likely be of most use to multi-lingual people who want a faster option to switch between languages. For an option that can translate existing online text, see the gTranslate add-on for Firefox.