Did you think MS was going to give you one for free?
Of course there are ways of obtaining product keys... but they are not exactly legal. With WGA, it's also a royal pain in the ass because MS keeps updating it, making it necessary to keep cracking it. Even if you do crack it, you still can't download a lot of things from MS's website (IE 7, Media Player 11, Windows Defender as examples)... so to me it's really not worth the hassle. I'd rather pay cash and be legit than to put up with all the BS.
By law you are supposed to buy a product key for each machine running. However ,I personally have called MS on vista and xp when needing a new product key. They verified that I had an actual cd, asked why i needed a new key and gave me one. Currently, I have one cd for vista and one cd for xp. But I have three vista product keys all work. The new key I was given for xp has been used on 5 machines. currently it is on 3. All are registered with MS. I dont condone pirating software but it can be done. MS WILL give you one (around 5)for free.
YES one cd will install on both machines. but it must be a real MS cd not a restore cd.
P.S. I bought the machines with xp pro. I just did a clean format because of all the junk that comes on a new computer. I OWN 7 or 8 product keys. I chose to use one because it is easier. If MS needs it I can prove every copy I have or should have. All machines are registered and have automatic updates on.