About the Windows 7 upgrade my friend Pete says:-
Easy to say in hindsight, but I've never performed a Windows upgrade and hopefully I never will. It's clean installs every time for me.
and of course he is right. He should be; ICT his job and he is damn good at it.
The temptation to upgrade rather than re-install comes from the fact that upgrade versions of Windows are cheaper and in theory an upgrade should save you hours of re-installing all your programs and data back once the new System is installed.
When you think about it though, there are millions of different configurations of Windows depending on what software and peripheral hardware is installed . It would only take a dodgy driver for something as innocent as a pen drive to upset the upgrade process.