You're asking two different questions here:
1. How to test software installations and deployments
and
2. How to assist a user with some issue
As a general rule, IT, or anyone else, should never, ever know a users password. Ever.
Consider this scenario:
User Joe. A bit disgruntled. He logs in and trashes the main inventory database. Costing twns of thousands of dollars.
Investigation shows that the 'Joe' account logged on and trashed the db.
All Joe's lawyer has to do is to point at the IT dept and say "Well...they know his password as well. Prove it was Joe."
Charges dropped.
For the above scenarios
1. Testing and deployments - the IT dept should have a small 'test network'. Just has to be a couple of machines.
Test and deploy on that, to see how things work.
You should never, ever use the users systems to test your deployments.
To actually deploy? AD and GPO.
Or log in remotely, as an admin account.
2. User assistance and troubleshooting - Go to their desk and watch them make the thing not work. Or set up some sort of screensharing deal.