Like I tell most people...
Go here: http://store3.esellerate.net/store/support/Default.aspx
... each time you reformat, re-install, etc. If you don't have your key handy you can get it by going there. Enter the e-mail you used to buy the game and it should give you your key. If it requires a valid working e-mail, just send a tech support e-mail with the new address (and old one) and include your invoice number.
That way you'll never need it. I don't recall if it saves to your computer. But if you ever need to re-install that's the way to do it. Then, KEEP the e-mail that you get so you wont ever lose it.