Re-entering key for XP

[Kursun]

A friend told me about his problem on the phone.

He has an old PC, running Windows XP.
After a system restore now he cannot do updates from Microsoft, being rejected because his OS is not a legal copy. He does have legal disk and key.

He is not too good with computers. Is there any easy way, without doing a repair, he can re-enter his key again?

 

[tigsounds]

Step-by-step and even a link to a pictorial is HERE

 

[Kursun]

Thank you.
Unfortunately, it seems, it's no easier than a repair re-installation for a computer novice.

 

[Kursun]

Best answer selected by Kursun.