I'm having the same problem. My serial number is for something Pre-SP3 and I
can't activate my completely legitimate license on my laptop

. I'll be
calling their licensing department in the morning and having them send me a
complimentary copy of Service Pack 3 Home full if they can't fix it without
forcing me to reinstall with a prior service pack. I slipstreamed SP3
thinking that was the smartest thing to do, it appears some stupidity (lack
of testing at Microsoft) has disproved that logic.