"W7 will show that message ... "
Does it now? I didn't know that. I am a relatively recent convert to Win7 (from XP) and I activate immediately upon installation.
I thought it just nagged you: "25 days to activate, 24 ...", and so on.
If you fail to activate it with the given timeframe, it will change to a black desktop with the message that it is not a genuine copy.