I few years ago I made the switch from mouse to track ball. I have had Carpal Tunnel for several years.
I need that software to make the extra buttons do something other than their default back/forward, but Windows handles the defaults just fine. The MX518 I'm using right now has 4, all of which work fine, without the custom software that came with it. They must also be doing something strange for that to be the case, as, like Ardax said, Windows has no problem with multi-button mice. Not that I've ever purchased anything from Kensington (they've always struck me as the budget version of Logitech/MS, when those two aren't too expensive in the first place), but if what you say is true, that's just sad. If Kensington is selling hardware right now that doesn't work on current operating systems, then shame on them. What I said obviously only applies to old hardware. The other two cannot be mapped for other functions. Only two of those buttons would work under Vista or Windows 7 and the trackball costs $75-$100. The fact is that there are current Kensington trackballs which have four buttons. They say the four button trackball is Vista certified but there is no mouseworks software available for Vista or Windows 7. So perhaps they should be labeled for XP/Mac. It works across all the Expert trackballs, the current ones with four buttons as well as the older ones with four buttons and an additional series of buttons at the very top. The mouseworks software works well, is highly customizable and one of the reason to use these Kensington trackballs. Thanks for the tough love even though it's way off base. The only trackball-using freak I know is my cousin, and her Logitech Mouseman just plain worked when I plugged it into her laptop after installing Windows 7 RTM. In the future, I'd suggest that it is unwise to expect weirdo hardware to be supported by any newer OS than whatever it supports right out of the box. So you've got a piece of custom hardware that was released almost six years ago (January 2004, as far as Google can see), and you're upset that it has exactly the same software support as it did when you bought it.
If Kensington never made Vista drivers for the thing, that rather implies that it's older than Vista.