WIN 7 UPGRADE VERSION - BIG WARNING TIME... TWICE OVER:
ONE: If you have XP on your machine, and buy the "upgrade" to Win 7, make very sure to copy off all your documents, valuable stuff, etc before you start. You cannot "upgrade" straight from one to the other - it will delete lots of stuff. Your documents may or may not survive if they are on drive C.
TWO: You have to leave the Win XP on the machine, or the Win 7 will refuse to validate. It expects to be installed over an earlier version of Windows, even though it will delete that. The previous process of "insert qualifying media" doesn't apply. It gets very complicated if you wipe the XP and start from a clean machine.
Don't ask how I know...