What about your wireless router? The firmware in your car? Your smartphone? Hell, even MicroSD cards run an embedded OS on an ARM processor to handle bad block remapping and to make it easier to test the cards before they leave the factory.
If you want to stick strictly to your desktop PC, let's talk about the software running on your network card, GPU, SSD, hard drive (some hard drives even have serial ports that you can connect to and see terminal output!).
I guarantee you that you use a *lot* more computers every day than you realize, and the vast majority of them run proprietary software.