Reinstalling Vista...

I'm helping a friend with his laptop. He's got major issues, he managed to get a trojan which has borked his PC. I've spent a good deal of time cleaning it all off for him, but he's still got a laundry list of problems. I suggested he simply reinstall Vista (a clean install) and he's ok with that.

Problem - he can't find the Vista disk that came with his laptop. I have one that came with mine, and we've got his serial number for his. BUT, they are different versions of Vista. He's got Vista Home Professional (I think - I don't have the laptop with me to look at), and I'm using Vista Home Basic.

So, can I install Vista Home Basic from my DVD onto his laptop using his Vista Home Pro serial number? What are the ramifications of doing so?

TIA.