Office for mac has determined that your product key is not valid

When launching Excel, Entourage, PowerPoint, Word, you have a message :
"Office for mac has determined that your product key is not valid"

Let me guess....this was probably right after you added the latest update ...
Microsoft Added this feature without informing anyone. You may have a valid School or University Licence, Still, MS would like you to buy it again ....

I knew that Office for Mac was a ClusterF*ck*, specially Excel but I am sometimes nostalgic too...I keep on looking at it..
So, here is how to fix your issue:

1) Time Machine*

Go back before the update, restore, et voila ! :-)
(I did a full restore of the /application/Office for Mac)
Save the latest v. of your .doc, .xls, .xlsx , etc files
Tip courtesy of Fran. Thank you Fran ;-)



 2 ) Manual
Removing Office 2008 License Files:
If your copy of Office 2008 was installed with an invalid product key, or if you mistakenly used the same product key to install Office 2008 on more than one computer, you must remove the invalid product key and enter a valid product key to use Office 2008.
1. In the Finder, select /Applications/Microsoft Office 2008/Office/OfficePID.plist, and then on the File menu, click Move to Trash.
2. Select /Users/username/Library/Preferences/Microsoft/Office 2008/Microsoft Office 2008 settings.plist, and then on the File menu, click Move to Trash.

-YOU WILL BE PROMPTED to RE-Enter your license, as used before the Update
 A Valid license key for Office 2008 looks like:

Please be aware that those two keys are given as "example" and you must NOT use them.
As they are valid, it would be illegal for you to use them, so, just don't use them.

W339P-JRCPB-XX8XD-YK49M-7DYCW
OR
FWCQQ-3XX2G-3CD89-4VRJK-CR4YD


3) Time Machine Again (simple)
This is an untested Solution
Restore files:
/Applications/Microsoft Office 2008/Office/OfficePID.plist
/Users/username/Library/Preferences/Microsoft/Office 2008/Microsoft Office 2008 settings.plist


Leave a comment if you have other solutions
Thanks
*Excel 2008 is extremely slow. Microsoft has received numerous complains about "stupid slow performances"
*Time Machine
If Time Machine is not set up on your Mac, this is a reminder: Time Machine WILL save your life.
Time Capsule: Apple just upgraded Time Capsule: for $299 , you have a 1 TeraByte backup disk AND an Airport Extreme base Station <<< good deal