In Windows 2000 or Windows XP1, assuming that you have the Accessibility Options installed, go to Start > Programs > Accessories > Accessibility > On-Screen Keyboard. It looks and works a lot like Key Caps on the Macintosh. I can choose between 101-key, 102-key, and 106-key (Japanese kana keyboard) layouts, although you may need to install the appropriate language options to get at more keyboards.
For an Accessibility option, it's not very accessible at first. You could always make a shortcut on the desktop, or pin it to the Windows XP Start Menu...
1 Typhen reports that Windows ME also includes this feature.