depends on your ethics.......
............but if you are in the UK then its definatly illegal to access peoples data without that persons, or asenior person (manager etc) permision under the DPA
If you are in the US, it's probably no illegal as the data belongs to the company, not to the person, and as a company employee you can view the companies data you have rights to. However that said, if you have rights to view someone else's data and you do so, you had better have a good reason for doing so. Most companies will fire a network admin for looking at someone else's data.
As the admin you are expected to be a model employee considering the rights that you have on the network, and the access that you have. If you are caught violating the trust the company has in you, you will have a very hard time getting a good reference from the company or any co-workers.
Odds are looking at data that you aren't suppose to be looking at will be a violation of the agreement you signed when you started working there.
The truth of the matter is, whatever employees don't want you to see, they will save on their machine, or on a CD, or ona flash driv. Yes, it is possible for you to view this if u remote desktop, but for the most part, users are wise enough to not put questionable material. I view this situation as a tricky question because you may feel it is a security issue. Even so, you can establish the security without intruding on other's rights. This being the case, there has to be other reasonable evidence or doubt necessary for looking at other's files.