I haven't found any official software that works for XP SP2. However, TweakUI which is part of Microsoft's PowerToys for XP will allow you to customize some of the buttons on the keyboard. Once you have it installed, run TweakUI and go to Explorer->Command Keys.
If you mostly use your buttons with Winamp, here are a couple of plugins for Winamp that work well with the SK-8135. The first is ready to go after installation. The second requires a bit of setup to get it working. Nothing I've found so far works with the volume knob though.