If you have to install Windows and you can not register online(CD is original) what to do????

Call the windows helpdesk to register it.

follow the prompts when it tells you to register windows and when it comes to register by mail or over the Internet or by phone choose register by phone a registration code should appear on your screen along with a toll free number after ringing them when they ask quote the numbers the operator ask for or he may give you a totally different number for you so either make sure sitting in front of your computer or you have a pen and paper handy to write it down. I'm assuming your windows version is XP.

If you mean Activation, then during the install it will ask to activate online OR over the phone with an 800 number.

Registration is not needed

You call them instead, it gives you that option.