XP product key lost

I went through lot of posts regarding this problem but couldnt find anything same as my problem. I bought a Dell with XP home OEM, then bought a XP pro. Now,

1. My hard disk is dead. (cant slave it, it died, and now I want to install XP pro on new hdd)
2. My product key, along with purchase reciepts is missing.
3. I only have the xp pro CD. (yeah, I misplaced the xp home cd as well, not that I wanted to install it.)

How do I install it?