Windows XP Home CD *lost*!

I have a friend in need of some help, he was forced to whipe his PC the other day due to a virus, now.. he didnt get a 'recovery' cd rom with his laptop he bought few years ago, so he thought he would download a copy of this from the internet & use his original CD-Key.. that way it would be ligit, but this didnt work. so he rang me asking for help.. because he cannot use his laptop because it wont accept his own CD-Key.. and as ironic as this sounds.. he doesnt want to have an ilegal operating system.. even though he downloaded a copy, he is a genuine guy & has a genuine windows key, but I was unable to assist him anymore because I, like him.. dont like doing things the ilegal way. he did say he would buy it again if he had to, but it would be alittle anoying because he got it pre-installed when he bought his laptop a few years ago. I know there is 101 ways to get this working ilegally. but is there any way LEGALLY?

so basically the question is..

Can he use his original CDKey with a copied windows CD does he need a special version? he doesnt have his own, so how would he get round this issue?

thx