All I know is that buying a second copy of Windows is an attractive option to swapping umpteen-haracter key codes with the MS rep. (I've done it twice, once after a string of upgrades and once after I gave my old copy of XP Home to my girlfriend)
The reps are congenial about it though, at least. The first time I expected a hassle, and they did ask casually why my key expired, but didn't press the issue. (It was completely legitimate in both cases.)
I think your first computer will eventually need to be re-activated though, as the key is now invalid. You could enter your new key to re-activate that PC, but after doing that you will eventually need to re-activate the second PC again. (You could probably limp along for a few months though, since you get 30 days to activate)
There are usually some decent deals in FS/FT on used copies, if you're looking for an alternative.
-Russ