MadData can verify your purchase by the email address used when origianally purchased. They can probably also verify by creditcard, and they have been very helpful to people who have lost keys in the past.
Least they did with me, which is why I learned my lesson and have a hardcopy OFF the computer now, and have the email archived on disc, and any other way you can think of of keeping it around.
I have never heard of them not helping anyone who has legitamately purchased the game.