gTrans is an interface for Google Translate. The free Windows application doesn't require installation, so you can use it from your USB drive.
gTrans sits in the system tray and lets you enter a text and a pair of languages. My favorite feature is "translate from clipboard": if you select this option, every time you copy text in the clipboard, gTrans translates the text for you.
Update. There's also a very nice Google Desktop plug-in that translates a text while you type it:
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