Registration is optional, Product activation is what you are referring to. The answer is no, there is no limit as to the number of times you can install and re-install and activate either RC2 or the final version. There is, however a limitation as to the number of hardware changes you can make before having to call MS and ask for a new Ident #. In other words you can change one of the four devices used to track WPA like the CPU as many times as you want, as long as the other devices used by WPA remains the same XP assumes this is the same machine. but if you change X amount of devices, which I believe total up to 4, within whatever period I think it's 6 months or so then XP thinks it's a new machine and has you activate it by phone. I've done it, it takes about 3 minutes.