I think marketing has a lot to do with the definition. An appliance (to me) is a branded product that comes to the consumer in a box with a pretty picture and a logo on it. You pull it out, plug it in, and have minimal configuration (that you can get support to help you with) to integrate it into your network. You update it with the updates the manufacturer grants you, and you can configure what the manufacturer allows you access to.
I would absolutely put a Windows PC in this category. Remember when some PC's used to come with the "break this seal and void the warranty" sticker on the back of the case? The point is that it's not intended to be a user-serviced item. To me, that's the defining characteristic.
If you want to compare it to cars, I would say that adding seat covers and custom rims to your new factory-original car doesn't really count as "modified". Swap the motor out, put a turbo and an intercooler on it, aftermarket, and now you've properly modded your car.