old windows cd key

[the coolest_123]

I am building a PC and want to put windows on it and I was wondering if I could use old windows 7 cd key to put windows 10 or 7 on my PC.

 

[slowhands95128]

Your key may possibly work but only for Windows 7, not Windows 10. Sometimes Microsoft will not accept an old Key for Windows 7, especially an OEM key from Dell, HP or other OEMs, or if the key has been de-activated because the system was upgraded to Windows 10. They may refuse entirely or they may ask you to do a phone activation. If they refuse entirely, the only legal option is to buy a new Windows 7 disk and Key on ebay or Amazon.