Can you tell me what options you're being given? You should still be able to activate online or over the phone.
If the options you're being given are...
1. Activate Windows online now --> (this will try to activate with whatever key you already have installed)
2. Buy a new product key online --> (self-explanatory)
3. Retype your product key --> (this is the same as "Change product key" and will give you the opportunity to enter a new key if you want to activate with a key that's not yet installed)
4. Show me other ways to activate --> (this is will give you two more options
a. Use my modem to connect directly to the activation service
b. Use the automated phone system
... please try phone activating by clicking on the last option, and then clicking on the last option in the next pop-up.
Let me know if that works. Thanks!