Hello and Welcome to FixYa!
I remember I was in need of the same thing, and the best ever I found was Team Viewer, got many versions, free premium, corporate according to your need. Free version got 2 types, a full and a quick view. Full version got all the features, like you can view other connected PC's but the Quick view can only be connected to full version.
You can download any of then from here
If you want to have a client service architect, means the client PC should be controllable and no need to control any other PC from that Client Machine, then Install the Full version on the Server side and Quick Support Version on all the Clients. Its just a tip, you better know your stance.
Every PC will have a Unique ID and pass-phrase by default (customizable).
I hope this will help you out, If you are having any questions/problem don't forget this place; FixYa.
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