For old timers, there's still a certain peace of mind that comes with a new install, especially on a computer that has been up and running for a while. Even if there's nothing apparently wrong with your Windows 7 installation, starting fresh can wipe out apps, settings, and other cruft that you no longer use or have simply forgotten about. There's a peace of mind in knowing you're starting with an operating system that is as clean as it can possibly be.
On the other hand, doing a clean install is a huge pain. You have to reinstall and reconfigure all of your applications, assuming you can find all of the requisite install media and product keys. You've got to track down drivers on manufacturer product pages that may or may not have even been updated with drivers for your computer. The install time you save versus doing an upgrade install will be more than wiped out by the time you spend getting everything just the way you like it.
Conventional wisdom usually says to do a clean install when upgrading your operating system, but operating systems have gotten better at dealing with upgrade installs. This charge is led by smartphones and tablets, which simplify even major updates by making them come down automatically over the Internet. We performed an upgrade install on a Windows 7 PC in daily use so we could see whether the stigma against upgrade installs is still applicable.
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