As long as your Windows 7 Retail disk matches the version of his COA (Home Premium, Professional, etc.), then you CAN activate it, but you must choose the option to activate by phone.  I've done this a sad number of times :)
I have bought and installed a retail version of the Win 7 OS.[Full Version copy] I am looking to reinstall Win 7 OS in my son's Dell. He'd lost the Dell's OEM disk. My question is can I install the Win 7 OS using my retail copy on my son's computer and then put in the the COA from the Sticker on his computer to activate the OS? Can I put in his COA number while installing the OS?   Or I have no option but to have find this OEM OS disk?   Regards
Can I install the OS from a Retail disk to activate OEM Version?