User blocking net admin access

I have a user that under the excuse of storing highly sensitive information in his ws, blocks my access as a net admin to the pc.

Of course, without any supervision, the ws was infected shortly with a virus.

After a full reinstall, he changed the Windows password once again.
Now my boss asked me about a lost file in this user’s pc.
He always refused to store any information on the server, that is backup nightly.

I’d like to hear from others in my same situation.

Thanks in advance,
Ernest