If it was an upgrade, then you should have the key either on the laptop or on the package that the disc came in. If it's a torrent copy, then you're foolish for installing it (and as was mentioned above, you won't get any help here). Otherwise call whoever you got the disc from for a key.
Have a great day:)
It should be noted also that if you installed without a key (as you can legally do for 30 days), you have to get the key for the version you installed (Home Premium, Business, or Ultimate). You can get a key for a higher level version (Home Basic can get Home Premium or Ultimate, and Business can get Ultimate), but if you want to get a lower version, you'll have to do a clean installation. The key won't downgrade.
by PatrickDickey: added information about upgrading/downgrading and keys