A program like magicjellybean will be able to get the Windows key for you. You have to use a program like this before you wipe the original partition or it won't work.
If you have already wiped the drive and started over...well you are sol
Just wanted to echo that while this is true, there is another twist. The original OEM install, is installed with an identical VLK OEM royalty key on all of that mfg's machines. Which means, that key cannot be used to activate a clean install either, unless you also save and migrate the OEM SLP stuff over to the clean install too.
Normally, the key printed on the sticker on the laptop is unique, and CAN be used to activate a fresh install.
If you've lost that key, or it has become unreadable, and you didn't make recovery discs, then you are indeed SOL.
Go buy another copy of Windows.