Need windows 7 home premium activation key?

i bought a dell inspiron 1545 from wal-mart that came with windows vista. shortly after, i was eligible for a free windows 7 upgrade. i recieved the upgrade in the mail. a couple months later, i installed ubuntu, a linux based operation system and uninstalled windows 7. i realized i no longer wanted ubuntu and wanted my windows 7 back, but i didnt have a "clean install" windows 7 disk (only the upgrade from vista disk.) because i was still under warranty dell sent me the windows 7 clean installation disk... but no activation key. i didnt think this would b a problem so i procrastinated asking for one. turns out im no longer under warranty, but need an activation key to set a background and whatnot. my computer says i may b using counterfeit software; which im obviously not. please help!