Network Security

I have a couple of quick questions im hoping someone will be able to help me out with.

Basically I use my own mac at work. Its the only mac in a room full of pcs, all on an ethernet network. I have to allow other users access to the computer in order to drop files on there.

This wouldnt be a problem but theres a couple of people there who are probably not the most trustworthy people in the world. Basically I want to prevent people from snooping around the computer, and just make sure its really secure.

At the moment i have created two user accounts, Giving people access to a restricted account when they need to drop things on the comp.

What other steps do you think i should take to make sure the security is tight?

I do things like check ebay and my email at lunch and want to make sure I dont leave any way to access passwords, or even my personal profile on the computer for that matter.

Thanks for the help!

Tom