It is unethical for him. Maybe it is acceptable for the company at large to spy on you and your coworkers, but unless it is explicitly in his job description to conduct surveillance on you, what he is doing to you is unethical and possibly illegal. At the very least that looks like a case of micromanagement.
As what you can do about it, besides consulting a lawyer, or finding a new job (his surveillance can certainly make an employee paranoid) you/your coworkers can tell the HR about his (suspected) activities, safely and discreetly. Then ask for some advice. But be careful. Be certain that they are on your side first. And be careful how you say it. Make sure you do not come across as accusing the administrator of spying or invading your privacy.
Meanwhile, what you can do is to not login any of your personal accounts to your computer. If you already have, log them out and erase the cache. That fellow may hold the information in that against you.
Do not use your company workstation to view sites that had nothing to do with your job, such as confession blogs, dating sites.
If you are a developer or designer, do not visit YouTube with your workstation, even to watch tutorials. Those kind of fellows tend to assume that you're watching non-work-related videos.
Do not search for terms that had nothing to do with your work using your workstation. The administrator might use the search information to advance his agenda.
Log out this account of yours from the same workstation.
View your personal accounts out of reach of the cameras. Do it outside the building if you have to.
Check out the questions posted in the comments as well.