OEM Windows versions will only activate with the specific OEM license key provided by the hardware manufacturer (Acer in this case). They will not activate with other retail Windows keys that you may own.
There should be a Windows hologram sticker on the case with the correct license key. Windows should activate with that key. If you call the Microsoft activation phone number, they should be able to look up the key immediately and tell you if it is a genuine key, or if that key has already been used to activate another machine.
If you have an OEM copy of Windows then normally as soon as you set up the machine and connect to the internet it will prompt you to register and activate Windows. This would happen as soon as you switch the machine on and go through the setup screens. Can you say how this process was somehow bypassed in your case? It may give some clue to what is happening.
A few years ago the disk on this Samsung netbook died (when out of warranty). I bought an identically sized disk, inserted it, and loaded WinXP from a completely legitimate CD, and used the WinXP product code on the bottom of this laptop. Installation was successful, but...
First boot dumped me in an unfamiliar Microsoft dosbox-like window which said, effectively, "shan't; not going to boot".
Samsung said it is a Microsoft problem; talk to Microsoft (quite reasonably since it was a MS message).
Microsoft said it is a Samsung problem; talk to Samsung.
Samsung would only sell me a third disk with XP preinstalled, cost disk+WinXP licence.
Microsoft didn't give a flying <expletive deleted> that I had already bought WinXP for this machine.
Consequently this machine now runs Xubuntu only, and I have no intention whatsoever of upgrading my main computer from XP to Win8 ( I might have done to Win7, but MS shot themselves in the foot with that one too!).
Microsoft is now in the same position that IBM was in in the early 80s: back then nobody cared about IBM because micros and workstations were clearly going to eat IBM's market.