Uhh...no...only way you're gonna get a "limited" amount is when you've reinstalled too many times, and it won't activate over the 'net...then you just have to call M$ and get them to activate it for you...
Use can use it on the same machine as many times as you want. It will only activate 3 times on 3 different machines. You can only have it on 1 machine and activated at any given time. This is the legal way.
I believe the key is good only s long as Microsoft keeps their activation servers up and running or to many people crack the key which ever comes first...