Your original post didn't say this. Backing up means copying the keys to diskette or burning them to CD. Like the email from slysoft said. Not storing them on the same system that might crash or get them deleted.
Did you order it over the internet? The key No. is in the email they sent you that gave you the link that allowed your keys to be downloaded. Go into your email queue and find that email either by using "Seach" to find emails from Elaborate Bytes (Elby) or Slysoft or (whichever you ordered from). Or scroll down to emails from the approximate timeframe you ordered and scroll till you find the email with the key information.
Or did you order by mail? You mentioned you had a receipt. That should have the info needed too.
So how was it ordered?