This will install the original game client.
The download will copy the file ac1install.exe to your downloads folder (Chrome)
Right click the installer and Run as Administrator. Running as an Administrator is just a habit of mine. I don't think it's really necessary.
Do not change the default install path!
If the installer won't run, delete it, and download again.
To verify the download completed:
Right Click the installer > properties > size of 571 MB is correct.