In Windows, a pop-up that reminds the user—at a frequency they can set—to restart the computer to install patches.
For example, a pop-up every 2 hours: “Hi! Please restart your computer to install the latest operating system updates”—or something similar—with “Restart” and “Later” as options. If necessary, include a setting that says, “After 10 prompts, only the ‘Restart’ button will appear.”