The original XBox was very similar to a PC, but the 360 not so much. The original XBox had a BGA 733Mhz Celeron processor and (essentially) a GeForce 3 GPU, and the controller ports were just fancy-fied USB ports. Of course everything that USB ports now, but that's more of a conformity than it is a sign that something is like a PC.
Anyway, PCs are so hard to fit into any definite category now, so it really isn't easy to say that something is "just a PC" or "is not a PC." Lots of things run Linux (for instance, every Android phone and tablet), but they aren't necessarily PCs. There are many HTPCs that use "non-standard" parts, but that doesn't make them a console.
The point is, I really have no point, and I'm just trying to make this more confusing. If it is running any type of PC operating system, then it would have to be Windows, however.
My guess, if it actually exists, is that it won't be anything like what people have been saying, and it will be either a Tegra 3 or Snapdragon 4 based piece of hardware, with maybe 4GB RAM and a huge hard drive, and will run a highly customized version of Android 4.0 (without access to the Market, and won't even use a GMail account)...but that's just my guess.