Thanks to Mackey D for pointing me in the direction of this aticle posted over at NVPlanet.

If you want to test out the new nVMixer on Windows XP 32-bit, then this is for you. We made a package available that includes the latest audio drivers from Windows Update and the nVMixer from the 64-bit nForce drivers set released today and tried it out on XP 32-bit. Warning: This is not not supported by nVidia nor by NVPlanet. Use at your own risk!!

Nvmixer 1.0 + Audio 4.31, how to XP 32-bit