I'd argue that Microsoft never actually built the MSXs, Microsoft more or less just spec'ed out what an MSX should be, similar to the way 3D0 never really built a machine (in fact I think some of the same companies who built MSXs built 3d0s if I'm not mistaken).

but...Microsoft did build the XBOX (which was a PC) and the XBOX 360 (which is less PCish, but still a computer).