Sometimes firewalls or routers blocking some of the installation traffic or other factors cause the online installer to fail. Using the offline installer usually resolves these issues. To get an offline installer, log in with your AltiumLive account on the downloads page: https://www.altium.com/products/downloads Underneath the Download link, click the link for "Need offline Installer?". After you tell us what issue you are trying to resolve by using the offline installer, you will receive an automated e-mail (it shouldn't take very long) letting you know to go back to the same page (usually ready for you in 20 minutes or less) where the old link will be replaced with a new link to download the offline installer. You will need to log out of AltiumLive and log back in again to see the change. This is a one-time process, so you won't need to repeat it when our next version is released. Here's a document with pictures: https://www.altium.com/documentation/altium-designer/offline-installation-of-altium-designer
If you still see the old link, usually signing out and back in causes the link to change. Clearing cache & cookies from your browser may be necessary. If you still don't see the Offline Installer link after clearing your cache & cookies and signing out and back in again, please contact support. We may have to reset your AltiumLive Account, and it's good to be in contact to be sure you can log in.
To save space, you can delete the unzipped folder after the installation is over. This is analogous to removing an installation CD. As with most software, Altium does not require the installation media to be available to run the program.