The product key could not be validated. Check the error log for details.

ok , can you check the log file and send the error .

is your new key from MSDN Team Suite Licence? If so you will need to call MS and ask them for a different key.

I checked the ULS Logs using the ULSViewer, There are no log messages. Event viewer too, does not show any messages. I tried multiple keys, but still the same message. I somewhere read that I should run the "Enable Features on Sites" found just above the "Convert License Type" option, but that isn't really working as well.

Attached is the screenshot of the log I see (ULS Log Entry seen using ULS Log Viewer)
ProductKeyIssue.JPG

Nope, my key is not from the MSDN Team Suite License.

did you use sharepoint setup with sp2 included.
I read a microsoft article about that (setup with sp2 included ) could cause this error.

I think I had installed SP2 later. Didn't install with it. (Installed had been done a month before where it was left as Trial and now when I am attempting to change it, I am faced with this!). What do you suggest?

i recommend to check with microsoft there is an update or hotfix about this

How do I do that?

Here are Miscrosoft support link,
http://support.microsoft.com/
http://support.microsoft.com/kb/295539

but tell me, did you try to upgrade from sp2 to sp3 (slipstream) or not, if not try it i think it will help you to solve this

sp3? I find that only WSS has Sp3, MOSS 2007 also has SP3? Please confirm.

yes both of them have SP3 (splitstream)

Do you have the links for the MOSS 2007 Sp3? I can only find WSS Sp3

Infact, I think the SP3 for WSS I found is for WSS 2.0 - http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=2bbfc89b-eb59-49ff-b58f-684693cb25a7 - So I do not think there is any SP3 for WSS 3.0 or MOSS 2007. Can you please point me to the links if there are available?