There have been a lot of questions about whether you need to purchase a full install package or an upgrade package when upgrading your machine to Windows 7. You will be able to perform a full install if you go through the custom settings so don’t hesitate buying the Upgrade Package and save the money if you already have a previous Vista or WinXP package on your machine. For more detail on procedures and lots of answers to nagging questions take a look at the following two blog posts:
Finally, some answers to Windows 7 upgrade questions
by Ed Bott TechRepublic