From Andrew Dunning:
Since the 1980s, Macintosh computers have shipped with a keyboard layout that makes it relatively easy to type diacritics and other symbols through the use of the Option key. This functionality has never been available on Windows, making it extremely cumbersome to type in foreign languages.
The "US Extended" keyboard layout is identical to the layout of the same name in Mac OS X, allowing you to type accents and special characters for foreign languages using easy-to-memorize keyboard shortcuts by substituting the AltGr key for the standard Option key. There is also a layout available replicating the classic US keyboard layout on the Mac.
Unlike the Windows "International" keyboard layout, which makes the quotation mark and apostrophe keys completely unusable, these layouts do not change the basic keyboard, and thus do not require changing any existing habits; they can thus be installed on a shared computer without causing any confusion. The On-Screen Keyboard in Windows will also allow you to view the combinations available.