Never upgrade any OS. Always purchase the full version. I am not talking
exclusively about XP/Vista/7 but from Windows 95 onwards
That's *terrible* advice, from two points of view:
1. The Upgrade version of any Windows is less expensive than the Full
Version, and therefore should be bought in preference to it when you
can.
It depends on what versions of Windows you are coming from and going
to, but in general the Upgrade version *can* do a clean installation.
For example, with XP, the requirement to use an upgrade version is to
*own* a previous qualifying version's installation CD, not to have it
installed. When setup doesn't find a previous qualifying version
installed, it will prompt you to insert its CD as proof of ownership.
Just insert the previous version's CD, and follow the prompts.
Everything proceeds quite normally and quite legitimately.
2. Although many people will tell you that formatting and installing
cleanly is always the best way to go, I disagree. Unlike with previous
versions of Windows, an upgrade to XP or later versions of Windows
replaces almost everything, and usually works very well.
My recommendation is to at least try the upgrade, since it's much
easier than a clean installation. You can always change your mind and
reinstall cleanly if problems develop.
However, don't assume that doing an upgrade relieves you of the need
to backup your data, etc. before beginning. Before starting to
upgrade, it's always prudent to recognize that things like a sudden
power loss can occur in the middle of it and cause the loss of
everything. For that reason you should make sure you have backups and
anything else you need to reinstall if the worst happens.