I never understood it either. After glancing over the wikipedia article I still don't.
http://en.wikipedia.org/wiki/.NET_Framework
Certainly *looks* like it's explaining something, though. Maybe my brain is too dense.
At any rate, I think it's 3.5 that's the 'catch all' installation.
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10101311 (20GB- backup empeg)
10101466 (2x60GB, Eutronix/GreenLights Blue) (Stolen!)