Even if you don't often install new programs, you'll inevitably encounter reboot requests when your system downloads software updates. On Windows machines, this is supposed to happen once a month on Patch Tuesday (the second Tuesday of the month), though Microsoft often issues "out-of-band" updates for major vulnerabilities. Macs don't have such a regimented schedule, but still receive regular updates for bug and security fixes. Linux is more variable, but platforms like Ubuntu which also manage application updates will have updates available almost any time you check.
In the area of updates, no platform is entirely immune from the reboot curse (though my own experience is that Linux pulls the trick pretty rarely). Windows cops the brunt of the criticism in this area in part because its popularity mean more people are aware of its foibles. Security researchers and criminals also spend more time trying to identify flaws within the platform, which in turn means updates can be more frequent. Macs don't have the same degree of rebooting insistence, but it's certainly not the case that they are reboot-request free.
But whatever the frequency, why does it happen? The standard answer goes something like this: installers and system software updaters often need to make changes to core operating system files, and they can't do that while the OS itself is in use without impacting on its stability and on programs that are already running. Rebooting lets the changes get made in a safe way without interfering with active tasks. (It also lets system security processes monitor changes to system files to ensure that unauthorised changes aren't made.)
Ideally, you'll always be asked before this happens and be given a chance to defer if you're busy. That said, Windows in particular has a nasty habit of force-installing updates at 3am in the morning without checking if you mind and then presenting you with a rebooted machine that has lost half your work the next morning. We've offered guidance before on how to disable this, but we don't recommend deferring updates indefinitely -- they're a vital and free part of keeping your system secure. We also don't recommend leaving your system for any period of time without saving any work-in-progress files.
Despite improvements in technology, there's no obvious end in sight for the reboot cycle. Microsoft itself has sometimes argued that well-written software shouldn't automatically require a reboot, especially if it follows guidelines for development on newer platforms such as Windows 7.
Yet although it has unrivalled access to Windows code, Microsoft appears to have difficulty practicing what it preaches, and its own software is often amongst the worst offenders, even when it is designed for quite trivial tasks. (The screenshot on this article is a reboot request from the latest version of Windows Live Essentials, for instance.)
Rebooting on Windows 7 is generally a much speedier process than on earlier versions, which can make the process less painful when it does happen. That said, if there's one thing guaranteed to make a reboot feel slower, it's an "Installing update 1 of x" message.
As computer usage moves increasingly into webapps and cloud interfaces, the notion of needing to reboot may seem more and more arcane. However, technology hardly ever runs smoothly, and I'd be very surprised if any device I own doesn't need some sort of restart (forced or otherwise) at some point in its life. In that context, being warned it is about to happen seems like positively good manners.
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