You can use the same cdkey in any computer activate by phone with the new MB. Thats all to it.
So i want to know, if i can reinstall windows how many times i want, even when i upgrade my pc? i know that i can only have 1 pc Active / license at the same time.
You can reinstall as many times as you like, but there is a likely possibility you might have to reactivate by telephone after the 5th time.
1) Press Windows key + R, then type: slui.exe 4
2) Next press the 'ENTER' key
3) Select your 'Country' from the list.
4) Choose the 'Phone Activation' option.
5) Stay on the phone (do not select/press any options) and wait for a person to help you with activation.
6) Explain your problem clearly to the support person. http://support.microsoft.com/kb/950929/en-us