For Mac OS there are two programs to do what you want, USB Overdrive and Gamepad Companion. I'm sure there must be something by now for Windows. Though i do know that when I looked a few years ago (because the MS software was even more useless then) there wasn't anything that worked with anything but 2 or 3 buttons (which, in my best New York accent, "is retaaded.")

Sorry that I can't give you the name of a specific Windows program. I'll keep an eye on this thread though because I too want to know. I'm quite used to the new Logitech mice, though I can only use the normal buttons, scroll wheel and thumb buttons. The other top buttons are completely useless unless you take your hand off the mouse.

Bruno