I was used to hear that from time to time it is needed to install your PC from scratch. From my experience this applies only to Windows PCs. My Linux machine at home "survived" already 2 major system upgrades which is comparable with getting your Windows 95 upgraded to Windows 2000 to Windows XP without having to setting it up from scratch over this time. The only difference is that there is not so much time between major version releases for Linux.
And what I lately noticed, when a Windows PC has to be installed from scratch (there is no real difference either for Windows 7 here): You loose a lot of time searching for and downloading the newest version of all the tools and applications you have installed and used over time. With the application repository (or repositories) on Linux this task is much faster (and can be either automated). Not talking about the fact that a lot of applications on Linux are already there with the default install without having to install them later. For one that does "just" E-Mail, Web-Browsing, Instant Messaging, writing documents, viewing and editing images, burning CDs/DVDs, viewing videos gets everything needed with the default installation (just for MP3 and such some codecs must be installed afterwards - at least for most distributions).
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