incorrect product key

I am hoping someone can help, I sent my computer to get some repairs done(lightning strike) and on return i am told i have an invalid product key, on speaking to the store i was told my XP pro disk was used and on my checking HKEY_local_machine\software\microsoft\windowsnt\current version the product key had two letters around the wrong way, i changed this but it has not changed the message i get, can anyone help please?

 

I know of instructions for properly changing your product key, however I'm new here, and I'm not sure on the policy for posting those instructions, so I hesitate because I'm sure some places frown on that regardless of the legitimate use.

 

Thanks, I'll use that as the guide for posting info I'm not sure about.