After developer Nick Lee realized that the Apple Watch's 520 MHz processor, 512 MB of RAM and 8 GB of internal storage made it more powerful than many desktops running Windows 95 in the 1990s, he felt confident he could get it to run Microsoft's successful operating system.
To get Windows 95 to run on the Apple Watch, Lee knew he couldn't rely on Apple's WatchKit SDK because it doesn't allow developers to directly access user touch locations. Instead, the SDK forces developers to use Apple's stock controls. So Lee had to patch certain files within a WatchKit app to load his own app code rather than Apple's. Lee tells MacRumors the process, which puts an x86 emulator into a self-contained Watch app, essentially turned Windows 95 into an an app.
Once Windows 95 was loaded onto the Apple Watch, the booting process took an hour because it's being emulated rather than virtualized. Lee also had to attach a straw to a small motor that nudged the Digital Crown periodically to keep the Watch awake. Once the Watch is all booted up and ready, users can control the mouse with their finger. However, because the emulation is so slow, Lee told MacRumors that "it only registers a few pixels per movement on the screen." To combat the lack of speed users can queue up commands by rapidly swiping on the display.
The Watch can be seen booting up and running Windows 95 in the video above, and Lee goes into further detail about the process in his Medium post.