There are a few direct things doable for work, but sometimes they don't fill your available time. So look for indirect things to do.
- Indirect can be preparation for the next tasks(I don't know anything about your admin job, but it can be making some scripts for checking everything is OK, or automating tasks).
- Indirect can be team improvement, like writing documentation, training people, etc. opportunities to do that can vary a lot between shops & positions. As you seem to be new, it's probably not much(cool for you if I'm wrong).
- Indirect can be self-improvement. Learning a new programmation language, checking the Workplace ;) , preparing yourself for any challenge that you could meet when you are not idle. Of course, it means you have access to a working computer during the install time. Depending on how your work is set up...
If not, try to have a small laptop for you, even an old crap, so that you can keep doing things while the install is running. If you can use it for things directly related to your job, perfect(as checking the network, the machines status, whatever).
NB : when it lasts only a few minutes, do not even try to be productive. The interrupt comes too quickly, and is highly demotivating. Otoh, when it's hours, you really have time to learn new things(like some kind of Linux scripting you haven't tried yet, or whatever).