Move Microsoft Office 2008 to new hard drive
Since you say it's a legitimate copy (I'm assuming retail—if this is volume licensing, that's a whole other kettle of fish), one path that's open to you is contacting Microsoft.

They are equipped to deal with lost product keys, and in most cases can help you retrieve it. Worst case, they can sell you a new one at a steep discount. 

See: How to locate your product key or obtain a new product key for Office for Mac.

(And for the future, you might want to stash your product keys and serial numbers away for safekeeping. 1Password has a category just for this. If you don't use 1Password, Keychain Access's "Secure Note" functionality can be put to good use.)