Go Win7, so many improvements - system speed, stability etc :)
As for SF and major changes like renaming - or adding another OS (dual boot) tends to trigger activations IMO.
And if you happened to spend another activation - no big deal.
When you run out, when you spent your last activation, just contact support and you'll get a new batch.
If you have more questions, we'll try to dig up some answers for you.
But I still have my original DCS & FC activations batches - and I do change alot of things on my PC's especially during testing.
IMHO, don't worry Vekkinho - just do it. :)
If you get in trouble, there's plenty of people who will help you.