Try installing twice. If you've performed a clean install from an upgrade disk then setup will not have recognised any legitimate existing OS being present.
Start Setup again, from within your unactivated W7 installation. When it has run it will have created a folder in your C:\ partition called 'Windows.old' and should activate second time over.
After it has activated you can delete 'Windows.old' and that's all there is to it, lots of people have discovered this works.